internal/lsp: add back check that a package was found for a given file

Added this check in golang.org/cl/161077 and unintentionally removed it in
golang.org/cl/161497.

Change-Id: I66bd2b616f4b41c25f134f6e9457c97bbcd3f72b
Reviewed-on: https://go-review.googlesource.com/c/162398
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/cache/view.go

@@ -121,6 +121,7 @@ func (v *View) parse(uri source.URI) error {
 		}
 		return err
 	}
+	var foundPkg bool // true if we found the package for uri
 	for _, pkg := range pkgs {
 		imp := &importer{
 			entries:         make(map[string]*entry),
@@ -150,12 +151,18 @@ func (v *View) parse(uri source.URI) error {
 				continue
 			}
 			fURI := source.ToURI(tok.Name())
+			if fURI == uri {
+				foundPkg = true
+			}
 			f := imp.v.getFile(fURI)
 			f.token = tok
 			f.ast = file
 			f.pkg = pkg
 		}
 	}
+	if !foundPkg {
+		return fmt.Errorf("no package found for %v", uri)
+	}
 	return nil
 }