compiler: set type information during traversal, fix #2231

Set all necessary context before file traversal, not only import
maps. Also, we can skip restoring import maps because all our code is
processed via `For*` iterators which set necessary context.
We can also refactor this a bit to have all context in one place,
this will be done in #2086.

Signed-off-by: Evgeniy Stratonikov <evgeniy@nspcc.ru>

pkg/compiler/testdata/nestedcall/call.go

@@ -28,3 +28,16 @@ func y() int {
 	tmp := 10
 	return tmp
 }
+
+// Token is stateless token.
+type Token struct{}
+
+// Method is a method.
+func (t Token) Method() int {
+	return t.Inner()
+}
+
+// Inner is a function to be called in Method.
+func (t Token) Inner() int {
+	return 2231
+}