gopls/internal/lsp/cache: better panic if metadata invariant is broken

As an invariant, we should always have metadata available for all packages in
DepsByPackagePath. Add a more useful panic for golang/go#61420.

For golang/go#61420

Change-Id: Ia4aa847163e54e6d3ba3d20b8ce14938d36087c5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/510701
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

Author: findleyr

gopls/internal/lsp/cache/check.go

@@ -807,6 +807,9 @@ func (s *snapshot) getPackageHandles(ctx context.Context, ids []PackageID) (map[
 		n, ok := b.nodes[idxID]
 		if !ok {
 			m := s.meta.metadata[id]
+			if m == nil {
+				panic(fmt.Sprintf("nil metadata for %q", id))
+			}
 			n = &handleNode{
 				m:               m,
 				idxID:           idxID,