x/tools/gopls: support the godebug directive in go.mod and go.work files

[findleyr]

This issue tracks minimal support for the godebug directive in go.mod and go.work files, added recently in #65573.

I think we need to:

1. Write (failing) tests that no diagnostics are produced in the presence of these directives.
2. Update x/mod, which should make these diagnostics go away.
3. (optional) Verify with a test that we surface diagnostics if godebug is used in go.mod files with older go versions.

[gopherbot]

Change https://go.dev/cl/588055 mentions this issue: gopls: initial support for godebug directive in go.mod and go.work