internal/lsp/cache: lock in snapshot.knownFilesInDir
internal/lsp/cache: lock in snapshot.knownFilesInDir

We were not locking while iterating s.files in snapshot.knownFilesInDir.
All other accesses of s.files appear to be locked, so this should fix
golang/go#43972.

Fixes golang/go#43972

Change-Id: I01184c3992c91f8beb4a3239f70cc4487a528ec0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/287573
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools