fix: Allow multiple cc_imports per pkgconfig file (#208)

Some packages, like `krb5-multidev`, include several .so files to link
against in their pkg-config files:
```
prefix=/usr
exec_prefix=${prefix}
libdir=${prefix}/lib/x86_64-linux-gnu/mit-krb5
includedir=${prefix}/include/mit-krb5

defccname=FILE:/tmp/krb5cc_%{uid}
defktname=FILE:/etc/krb5.keytab
defcktname=FILE:/etc/krb5/user/%{euid}/client.keytab

Name: mit-krb5
Description: An implementation of Kerberos network authentication
Version: 1.20.1
Cflags: -isystem ${includedir}
Libs: -L${libdir} -lkrb5 -lk5crypto -lcom_err
Libs.private: -lkrb5support
```

So, instead of assuming that we have one cc_library per pkgconfig file,
we just collect all the declarations and create one `cc_import` target
for each.

Some design considerations:

### Does this violate the private-ness of `Libs.private`?

All `cc_imports` we create have private visibility already, so I don't
think it does.

### Why not depend on other debian packages exporting those libraries?

Since those files are already in the package
(https://packages.debian.org/sid/amd64/krb5-multidev/filelist), I assume
the intention is to link against the `.so` files distributed in the
`krb-multidev` package, and not to pull other deb packages. This could
be false, of course, if a user has already installed those packages and
for some reason the linker finds their SO files first, but I think
that's a bug in the definition of the krb5-multidev package, not this
implementation.

Plus, it has the added benefit that, if the debian package in question
doesn't include an appropriate .so file, we just don't create the
cc_import.

apt/private/deb_import.bzl

@@ -272,36 +272,40 @@ so_library(
             includes += pkgc.includes
             link_paths += pkgc.link_paths
 
-            if not pkgc.libname or pkgc.libname + "_import" in import_targets:
+            if len(pkgc.libnames) == 0:
                 continue
 
-            subtarget = pkgc.libname + "_import"
-            import_targets.append(subtarget)
-
-            # Look for a static archive
-            # for ar in a_files:
-            #     if ar.endswith(pkgc.libname + ".a"):
-            #         static_lib = '":%s"' % ar
-            #         break
-
-            # Look for a dynamic library
-            IGNORE = ["libfl"]
-            for so_lib in so_files:
-                if pkgc.libname and pkgc.libname not in IGNORE and so_lib.endswith(pkgc.libname + ".so"):
-                    shared_lib = '":%s"' % so_lib
-                    break
+            for libname in pkgc.libnames:
+              if libname + "_import" in import_targets:
+                continue
 
-            build_file_content += _CC_IMPORT_TMPL.format(
-                name = subtarget,
-                shared_lib = shared_lib,
-                static_lib = static_lib,
-                hdrs = [],
-                includes = {
-                    "external/.." + include: True
-                    for include in includes + ["/usr/include", "/usr/include/x86_64-linux-gnu"]
-                }.keys(),
-                linkopts = pkgc.linkopts,
-            )
+              subtarget = libname + "_import"
+              import_targets.append(subtarget)
+
+              # Look for a static archive
+              # for ar in a_files:
+              #     if ar.endswith(pkgc.libname + ".a"):
+              #         static_lib = '":%s"' % ar
+              #         break
+
+              # Look for a dynamic library
+              IGNORE = ["libfl"]
+              for so_lib in so_files:
+                  if libname and libname not in IGNORE and so_lib.endswith(libname + ".so"):
+                      shared_lib = '":%s"' % so_lib
+                      break
+
+              build_file_content += _CC_IMPORT_TMPL.format(
+                  name = subtarget,
+                  shared_lib = shared_lib,
+                  static_lib = static_lib,
+                  hdrs = [],
+                  includes = {
+                      "external/.." + include: True
+                      for include in includes + ["/usr/include", "/usr/include/x86_64-linux-gnu"]
+                  }.keys(),
+                  linkopts = pkgc.linkopts,
+              )
 
         build_file_content += _CC_LIBRARY_TMPL.format(
             name = target_name,

apt/private/pkgconfig.bzl

@@ -82,7 +82,7 @@ def process_pcconfig(pc):
     includes = []
     link_paths = []
     defines = []
-    libname = None
+    libnames = []
 
     IGNORE = [
         "-licui18n",
@@ -105,8 +105,8 @@ def process_pcconfig(pc):
                 link_paths.append(linkpath)
                 linkopts.append("-Wl,-rpath=" + arg.removeprefix("-L"))
                 continue
-            elif arg.startswith("-l") and not libname:
-                libname = "lib" + arg.removeprefix("-l")
+            elif arg.startswith("-l"):
+                libnames.append("lib" + arg.removeprefix("-l"))
                 continue
             elif arg in IGNORE:
                 continue
@@ -118,7 +118,9 @@ def process_pcconfig(pc):
             if arg in IGNORE:
                 continue
             elif arg.startswith("-l"):
-                linkopts.append(arg)
+                # The cc_imports we create based on these names are private already,
+                # so we don't need to do anything special for `Libs.private`.
+                libnames.append("lib" + arg.removeprefix("-l"))
 
     if "Cflags" in directives:
         cflags = _trim(directives["Cflags"]).split(" ")
@@ -148,14 +150,14 @@ def process_pcconfig(pc):
             "/usr/include",
         ]
 
-    return (libname, includedir, libdir, linkopts, link_paths, includes, defines)
+    return (libnames, includedir, libdir, linkopts, link_paths, includes, defines)
 
 def pkgconfig(rctx, path):
     pc = parse_pc(rctx.read(path))
-    (libname, includedir, libdir, linkopts, link_paths, includes, defines) = process_pcconfig(pc)
+    (libnames, includedir, libdir, linkopts, link_paths, includes, defines) = process_pcconfig(pc)
 
     return struct(
-        libname = libname,
+        libnames = libnames,
         includedir = includedir,
         libdir = libdir,
         linkopts = linkopts,