internal/proxy: use module path from go.mod in txtar file
internal/proxy: use module path from go.mod in txtar file

Rather than generating a module path that always begins with
"example.com", readTxtarModule now reads the module path from the go.mod
section of the txtar file when one is present.

Change-Id: I77ad7cded2a25da0eeb996f0a465be28120a887e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/314531
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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