cmd/compile: remove structpkg global variable

cmd/compile: remove structpkg global variable

The structpkg global variable was only used to verify internal
consistency when declaring methods during import. Track the
value in the parser and binary importer directly and pass it
to the relevant function as an argument.

Change-Id: I7e5e006f9046d84f9a3959616f073798fda36c97
Reviewed-on: https://go-review.googlesource.com/20606
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>