cmd/go: maybe show legacy tags in pseudoversions

[rsc]

If you do

go get foo@tag

where tag is not a proper semver tag for foo, then vgo looks up tag, resolves the commit, finds the date on the commit, and records instead of tag a pseudo-version like

v0.0.0-20180501123456-1234abcdef

I wonder if we should preserve the tag, so that the recorded version would be:

v0.0.0-20180501123456-1234abcdef-tag

This would work for branch names and legacy semver tags too, of course, so that we could have:

v0.0.0-20180501123456-1234abcdef-devbranch
v0.0.0-20180501123456-1234abcdef-v17.0.0

These would just show a little bit more information when you list your dependencies in projects not using tagged releases (or at least vgo-compatible tagged releases in the case of the legacy v17).

[myitcv]

Particularly useful if you're using branches during active development.

[rsc]

I'm not going to do this for Go 1.11. I think it's important not to clutter the pseudo-version with non-essential information, so that there is always a single canonical form in all the things that print versions. For Go 1.12 maybe we can find a way to put it in a comment instead.