internal/testing/pagecheck: rename packageURLPath to unitURLPath
internal/testing/pagecheck: rename packageURLPath to unitURLPath

packageURLPath is renamted to unitURLPath, since it returns the url path
for all unit pages.

Change-Id: I9b85d5eda8a5a9276b2b7b2ac370b8159405cce1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/277694
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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