go/types: enforce Check path restrictions via panics

Its godoc says that path must not be empty or dot, while the existing
implementation happily accepts both.

Change-Id: I64766271c35152dc7adb21ff60eb05c52237e6b6
Reviewed-on: https://go-review.googlesource.com/38262Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>

src/go/types/api.go

@@ -347,6 +347,9 @@ func (init *Initializer) String() string {
 // file set, and the package path the package is identified with.
 // The clean path must not be empty or dot (".").
 func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
+	if path == "" || path == "." {
+		panic(`path must not be "" or "."`)
+	}
 	pkg := NewPackage(path, "")
 	return pkg, NewChecker(conf, fset, pkg, info).Files(files)
 }