pkgtools/pkglint: don't access the internet during the build

Pkglint needs goyacc, which is provided by devel/go-tools. But that
packages pulls in a lot of dependencies that are not needed for pkglint.
This includes a Perl interpreter and various cryptography packages.

Instead, during build, pkglint installs the only needed tool of that
collection (goyacc).

Since go-package.mk assumes that each pkgsrc package only downloads and
installs a single project, but pkglint now needs two of them (goyacc and
pkglint), most parts of go-package.mk had to be copied here and
implemented directly.

(rillig)