avoid panic when embedding a builtin alias

TypeName.Pkg is documented as:

    Pkg returns the package to which the object belongs.
    The result is nil for labels and objects in the Universe scope.

When a struct type embeds a builtin alias type, such as byte,
this would lead to a panic since we assumed we could use the Pkg method.

Fixes #798.

testdata/script/syntax.txtar

@@ -244,6 +244,13 @@ var _ = embeddingAliasSameName{
 	Reader: nil,
 }
 
+type embeddingBuiltinAlias struct {
+	byte
+}
+
+var _ = embeddingBuiltinAlias{3}
+var _ = embeddingBuiltinAlias{byte: 3}
+
 -- external/external.go --
 package external