Go modules resolution should use modules path location instead of path declared in mod file

This is especially important for GoCenter, where users can submit project forks to be processed without fixing the path declared in the mod file, so the original path is still being used. Artifactory sees the module published as being the original one.

Example: https://gocenter.jfrog.io/gocenter/webapp/#/artifacts/browse/tree/Properties/gocenter-build/github.com/millergarym/opts/@v/v1.1.1.mod

github.com/millergarym/opts is a fork of github.com/jpillora/opts. The module name declared in the go.mod file was not fixed. After processing github.com/millergarym/opts@v1.1.1 the following request works.:

→ curl --head https://gocenter.jfrog.io/gocenter/api/go/gocenter-uber-virtual/github.com/jpillora/opts/@v/v1.1.1.mod
HTTP/1.1 200 OK

→ curl https://gocenter.jfrog.io/gocenter/api/go/gocenter-uber-virtual/github.com/jpillora/opts/@v/v1.1.1.mod
module github.com/jpillora/opts

go 1.12

require github.com/posener/complete v1.2.1

It should return a 404 instead since the original module has not been processed yet. This does not allow us to process the original module using the CLI because it sees the original module as already present there.