x/pkgsite: can get stuck in a redirect loop with a weird module path

[mvdan]

Take https://github.com/ipld/go-codec-dagpb, and note that the module path is github.com/ipld/go-codec-dagpb/dagpb, which has an extra /dagpb. That was probably a mistake or a misunderstanding of some sort, but that's not the point of this issue.

The point of this issue is simple: if you navigate to https://pkg.go.dev/github.com/ipld/go-codec-dagpb/dagpb, it redirects forever, presumably because it is confused by the mismatch :)

I don't think this weird edge case is particularly important, but pkgsite should never enter into redirect cycles. A good way to detect these in general might be a good idea, with plenty of tests.

cc @julieqiu @jba

[gopherbot]

Change https://golang.org/cl/295549 mentions this issue: internal/frontend: fix redirect loops