x/tools/gopls: disable package-oriented diagnostics for files/directories whose names begin with "."/"_"

[hyangah]

Gopls version: v0.12.0

How to reproduce

1. Clone and open the delve project (github.com/go-delve/delve). There should be no diagnostics.
2. Open one of the files in _fixtures. Gopls is publishing ~570 compiler errors (mostly, DuplicateDecl)
3. Close the file. The diagnostics don't go away.

The directory _fixtures contain various files used for debugging. Depending on debugging scenario, mostly a single file is chosen to be built and debugged by tests. They are like testdata.

According to the go command line doc https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns files in this directory are handled specially. But gopls's behavior doesn't seem to align with it.

Directory and file names that begin with "." or "_" are ignored by the go tool, as are directories named "testdata".

I'd appreciate completion or single file mode diagnostics on the files, but not DuplicateDecl diagnostics.

Alternatives to consider

• Provide an option to disable diagnostics on files in certain directories. -> Should this be editor's option?
• Unpublish diagnostics once the files in _fixtures are all closed. -> Still distracting.
• Ask users to use build tags. (//go:build ignore) -> at least, for delve, this will be a huge change since most tests are sensitive to file line number changes (e.g. setting breakpoint).