compiler: do not emit code for unused imported functions

Our current algorithm marks function as used if it is called
at least ones, even if the callee function is itself unused.
This commit implements more clever traversal to collect usage
information more precisely.

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

pkg/compiler/testdata/nestedcall/inner/call.go

@@ -0,0 +1,6 @@
+package inner
+
+// Return65 returns 65.
+func Return65() int {
+	return 65
+}