internal/frontend: fix links for subdirectories
internal/frontend: fix links for subdirectories

Links in the subdirectories section are now generated using linkVersion,
which fixes links for the stdlib.

Change-Id: I4dae002f604ff2bb47fbb8d6ae374516bada1804
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260037
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite