x/pkgsite: documentation broken for non-leaf packages on a branch

[hdonnay]

What version of Go are you using (go version)?

N/A

What did you do?

Going to a non-leaf package on pkg.go.dev on a branch doesn't render any documentation items. For example, https://pkg.go.dev/net@master and https://pkg.go.dev/testing@master#F.
I think this is a regression, as the latter link is in the fuzzing beta blog post.
Navigating to a leaf package seems to work, e.g. https://pkg.go.dev/net/url@master.

[hdonnay]

This seems to be fixed now.