gopls/internal/lsp/cache: request EmbedFiles from go/packages

`Gopls` attempts to diagnose incorrect embed patterns. The logic to
do so relies on `go list` to actually resolve those patterns and to
propagate an error in case the pattern is incorrect, see also [1].

`Gopls` explicitly specifies the needed JSON fields towards
`go list -json` which internally skips evaluation steps for unneeded
fields. Currently, the embed patterns are always evaluated but there
is plans to only run pattern evaluation if needed [2].

So, explicitly ask for EmbedFiles to prepare for [2].

[1]: https://github.com/golang/tools/blob/193023cca0b707b4d668e5d332649cba285e99f7/gopls/internal/regtest/misc/embed_test.go#L26-L40
[2]: golang/go#58522

gopls/internal/lsp/cache/snapshot.go

@@ -385,6 +385,7 @@ func (s *snapshot) config(ctx context.Context, inv *gocommand.Invocation) *packa
 			packages.NeedDeps |
 			packages.NeedTypesSizes |
 			packages.NeedModule |
+			packages.NeedEmbedFiles |
 			packages.LoadMode(packagesinternal.DepsErrors) |
 			packages.LoadMode(packagesinternal.ForTest),
 		Fset:    nil, // we do our own parsing