x/tools/gopls: support go.mod files

[stamblerre]

Context: Beginning of #31712.

Some other features that fall under the umbrella of this issue:

• Detection of the local module (x/tools/gopls: group imports together for the local module #32049)
• textDocument/documentLink taking the user to the exact version of their dependency (x/tools/gopls: support pkg.go.dev in textDocument/documentLink #35563).