x/tools/gopls: NarrowestMetadataForFile bug reported by telemetry

[adonovan]

This stack IsVbqA was reported by telemetry:

cache.(*snapshot).load:118 is

	if standalone && len(pkgs) > 1 {
		return bug.Errorf("internal error: go/packages returned multiple packages for standalone file")
	}

gopls/bug
golang.org/x/tools/gopls/internal/bug.report:35
golang.org/x/tools/gopls/internal/bug.Errorf:2
golang.org/x/tools/gopls/internal/lsp/cache.(*snapshot).load:118
golang.org/x/tools/gopls/internal/lsp/cache.(*snapshot).MetadataForFile:36
golang.org/x/tools/gopls/internal/lsp/source.NarrowestMetadataForFile:1
golang.org/x/tools/gopls/internal/lsp.(*Server).diagnoseChangedFiles:28
golang.org/x/tools/gopls/internal/lsp.(*Server).diagnoseSnapshot:25
golang.org/x/tools/gopls/internal/lsp.(*Server).diagnoseSnapshots.func1:2
runtime.goexit:0

golang.org/x/tools/gopls@v0.14.1 go1.21.3 darwin/arm64 (1)

Reported using golang.org/x/tools/gopls/internal/telemetry/cmd/stacks.

[gopherbot]

Change https://go.dev/cl/557635 mentions this issue: gopls/internal/lsp/cache: don't report a bug for standalone test files