Mk/Uses/go.mk: Default to "." instead of PORTNAME

If neither GO_PKGNAME or GO_TARGET are set explicitly, go install
tries to install a package named ${PORTNAME} which is almost always
wrong. This forces Go ports to set GO_PKGNAME or GO_TARGET although
their value is often redundant.

Update do-build to default to the "." package instead ("the package
in the current directory") and teach do-install to install the "."
package. In many cases, this removes the need to explicitly set
GO_PKGNAME and GO_TARGET making USES=go:modules alone sufficient
for simpler ports.

This is a backward compatible change.

• Assign maintainership to Dmitri

Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: jlaffaye (maintainer timeout, ~3 weeks)
Differential Revision: https://reviews.freebsd.org/D20813