chore(v2): vendor go/pkgsite/source package for remote URL discovery

[Bobgy]

Part of #73

I've already verified this module works perfectly for our usecases in following up PRs.

I asked whether pkgsite can move the internal package out, but I got reply that we need to vendor it, golang/go#40477 (comment).

[wlynch]

Let's put this in internal/third_party - we don't want to take on responsibility of maintain this package for others.

[Bobgy]

Hi @wlynch, I updated the PR to

• move the vendored module to internal
• vendor tests as well
• remove unused functions to reduce the size of the vendored files (they reduced a lot)

Note that, I did these in clear and separate commits, each commit does just one thing.

[Bobgy]

Updated to target master branch in root folder.