Emit shell warning if running outside of all roots

To help make it clear why Cargo may not load all ancestor configs or
find workspace manifests in ancestor directories, Cargo emits a warning:

```
warning: Cargo is running outside of any root directory, limiting
loading of ancestor configs and manifest
```

src/bin/cargo/cli.rs

@@ -9,6 +9,7 @@ use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::ffi::OsString;
 use std::fmt::Write;
+use tracing::warn;
 
 use super::commands;
 use super::list_commands;
@@ -132,6 +133,10 @@ pub fn main(gctx: &mut GlobalContext) -> CliResult {
     // Reload now that we have established the cwd and root
     gctx.reload_cwd()?;
 
+    if gctx.find_nearest_root(gctx.cwd())?.is_none() {
+        gctx.shell().warn("Cargo is running outside of any root directory, limiting loading of ancestor configs and manifest")?;
+    }
+
     let (expanded_args, global_args) = expand_aliases(gctx, args, vec![])?;
 
     let is_verbose = expanded_args.verbose() > 0;

src/cargo/util/context/mod.rs

@@ -639,7 +639,7 @@ impl GlobalContext {
         }
     }
 
-    fn find_nearest_root<P: AsRef<Path>>(
+    pub fn find_nearest_root<P: AsRef<Path>>(
         &self,
         start: P,
     ) -> CargoResult<Option<(PathBuf, PathBuf)>> {