internal/lsp: modify check for a missing package

This change moves from marking a package with `go list` errors as
missing, to marking a package with no files as missing.

Change-Id: Ibad1e67518d8a7f4c4bde416c53ab8132ae534e3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/184039
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/lsp/cache/load.go

@@ -118,7 +118,8 @@ func (v *view) checkMetadata(ctx context.Context, f *goFile) (map[packageID]*met
 			return nil, pkg.Errors, fmt.Errorf("package %s has errors, skipping type-checking", pkg.PkgPath)
 		}
 		for importPath, importPkg := range pkg.Imports {
-			if len(importPkg.Errors) > 0 {
+			// If we encounter a package we cannot import, mark it as missing.
+			if len(importPkg.CompiledGoFiles) == 0 {
 				if f.missingImports == nil {
 					f.missingImports = make(map[packagePath]struct{})
 				}