x/pkgsite: source files links for stdlibs are incorrect in local mode

[hyangah]

cmd/pkgsite version @ 0a30e374544fc794cc1769dd04254f5be9b62c68 (2023-03-01T13:51:50Z)

Repro:
run pkgsite
visit http://localhost:8080/time (it may take some time to index stdlib)
From "Source Files" section, click "format.go" and see the link is invalid.

I started pkgsite from my pkgsite project directory checked out at /Users/hakim/projects/pkgsite, and the link is "http://localhost:8080/files/Users/hakim/projects/pkgsite/golang.org/x/pkgsite/time/format.go", a strange combination of the process wd + main module path + "/time/format.go".

Probably related: the "Repository" info is incorrect.

[bcmills]

Possibly related to (or the same underlying cause as) #57742?