x/tools/gopls: provide a better error message when a file is excluded due to a package name mismatch

[findleyr]

Context:
https://www.reddit.com/r/golang/comments/10rto2n/how_to_work_with_multiple_go_files/

We can and should do better here: if a file is not included in any packages due to its package name, we should diagnose this and try to explain to the user why the file was not in any package.