x/pkgsite: support viewing a package @ default branch HEAD

Previously, #36811 added support for viewing packages at master on pkg.go.dev. Many repos are moving their default branch names away from master to main or something-else-not-master.

It seems that master is hard-coded in pkgsite.

Would it be possible to support some kind of magic version string that gets the default branch, whatever it may be named? This may end up being the reason that I move some of my repos back to master, which I'd like to avoid.

See also this comment as FYI

cc @julieqiu