gopls/internal/lsp/source: KnownPackagePaths: avoid loading

This operation is logically just about metadata.
This change makes it avoid package loading.

(This is a warm-up exercise for more invasive variants
in many other places.)

Change-Id: I408c754f10665d3959ee0f4ff6cf95c54ec94f3e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/452056
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/gopls/internal/lsp/cache/snapshot.go b/gopls/internal/lsp/cache/snapshot.go
index 28d6fff..8b02dea 100644
--- a/gopls/internal/lsp/cache/snapshot.go
+++ b/gopls/internal/lsp/cache/snapshot.go
@@ -1249,10 +1249,9 @@
 	return match
 }
 
-func (s *snapshot) getMetadata(id PackageID) *source.Metadata {
+func (s *snapshot) Metadata(id PackageID) *source.Metadata {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-
 	return s.meta.metadata[id]
 }