Convenience changes for Go

Hi,

Here is a diff that adds:

- -trimpath to MODGO_FLAGS: This removes paths like
"/build/pobj/blablablablbal" from the resulting binary.
- Teaches modgo-gen-modules-helper how to pass a version to the
get_dist_info stuff.
- Adds a new make target: "modgo-gen-current-modules". This will
generate the MODGO_ bits for the currently defined
MODGO_MODNAME. (existing target generates them for what ever Go
things the latest version of a MODGO_MODNAME is).

The -trimpath is mostly cosmetic, but it also gets us a bit closer to
"reproducible builds". This would potentially let us produce the same
binaries that an upstream does for releases (assuming build flags and
'-X' things are all the same).

The new target lets one bump a port to an explicit version (i.e. not the
latest, or a latest that has not yet propagated through the Go module
ecosystem).

Thoughts? Cluesticks? OKs?

Cheers,
Aaron