cmd/compile: fix names of inlined methods from other packages

cmd/compile: fix names of inlined methods from other packages

Previously, an inlined call to wg.Done() in package main would have the
following incorrect symbol name:

    main.(*sync.WaitGroup).Done

This change modifies methodname to return the correct symbol name:

    sync.(*WaitGroup).Done

This fix was suggested by @mdempsky.

Fixes #19467.

Change-Id: I0117838679ac5353789299c618ff8c326712d94d
Reviewed-on: https://go-review.googlesource.com/37866
Reviewed-by: Austin Clements <austin@google.com>