x/tools/gopls: eliminate dependence on "seen files"

[findleyr]

Caveat: this is an abstract issue, but sufficiently fundamental to warrant independent tracking.

In two places inside gopls, there is a notion of "previously seen" files that affects gopls behavior:

• cache.View.knownFile, which is used to determine if a file is relevant to a given view
• cache.snapshot.FindFile (and anything that walks the snapshot.files set), which is used to filter to relevant or active files

These notions have no intrinsic definition: by construction they are path dependent. As such, they have been a source of historical and current bugs (for example, changing configuration can invalidate the set of known files, which leads to undefined behavior). They are also a source of test flakes, which is really just a manifestation of underlying bugginess.

We should replace these with well-defined notions, on a case by case basis. For example "is a file in the workspace directory", or "is this file in a loaded package, after loading completes".

CC @adonovan @pjweinb

[gopherbot]

Change https://go.dev/cl/459560 mentions this issue: gopls/internal/regtest: avoid race in TestSwitchFromGOPATHToModuleMode

[gopherbot]

Change https://go.dev/cl/468775 mentions this issue: gopls/internal/lsp/source: use metadata files for workspace symbols

[gopherbot]

Change https://go.dev/cl/525616 mentions this issue: gopls/internal/lsp/cache: simplify tracking of snapshot directories