Allow USES=go ports to override make targets & include <bsd.port.pre.mk>

Prior to this change, if a port had:

• USES=go.mk,
• .include <bsd.port.pre.mk>, and
• overrode a make target (e.g. post-extract:)

then the following warnings were produced when invoking make on the port:

make: "/usr/local/poudriere/ports/default/CATEGORY/PORT/Makefile" line XX: warning: duplicate script for target "post-extract" ignored
make: "/usr/local/poudriere/ports/default/Mk/Uses/go.mk" line 68: warning: using previous script for "post-extract" defined here

This change fixes this issue by wrapping the definition of make targets
in a separate once-only !defined wrapper which is blocked until post.mk.
This concept was copied from Mk/Uses/python.mk.

PR: 224948
Submitted by: woodsb02
Approved by: jlaffaye (maintainer)