Updates package_files to filter empty strings

[JhossePaul]

I found an error while trying to compile an roxygenized package in Windows when the Collate directive was added to the DESCRIPTION file. The problem is that the package_files function returns the path to the directory if there are leading whitespaces in the DESCRIPTION file.

I would recommend to change the scan function for regular expressions as they are safer and have full control under the string.

For example, if

desc <- list(Collate = "'bar.R'\n    'foo.R'")
collate <- scan(text = desc$Collate, what = "", sep = " ")
collate
[1] "bar.R" ""      ""      ""     ""    "foo.R"

then;

> file.path("path_to_package", "R", collate)
[1] "path_to_package/R/bar.R" "path_to_package/R"       "path_to_package/R"       "path_to_package/R"      
[5] "path_to_package/R"       "path_to_package/R/foo.R"

If we pass this paths to the tokenize_file function, it will throw an error. I propose:

unlist(strsplit(gsub("'", "", desc$Collate), "\\n\\s*"))
[1] "bar.R"  "foo.R"

[hadley]

Can you please start by filing an issue describing the problem?