Log Message:
Use the non-symlinked go tool for package builds.

Otherwise, you will not be able to build Go code in pkgsrc if you do not
have pkg_alternatives installed.

Introduce a variable (user-settable for now) GOVERSSUFFIX.
$GO is now the Go tool with full path.

leot@ convinced me to go full Python (e.g. one directory per major version,
GO_VERSION_ACCEPTED etc.) but that will be another series of commits.