x/pkgsite: can not show dependencies with vendor directory

[aarzilli]

pkgsite when run locally can not show packages outside of the main module if there is a vendor directory. For example

$ git clone github.com/go-delve/delve
$ cd delve
$ pkgsite

and open http://localhost:8080/github.com/hashicorp/golang-lru/simplelru

[findleyr]

The problem is related to the fact that pkgsite fetch is based on modules, and vendored packages lack module info.

Therefore, this is part of a larger project to allow cmd/pkgsite to refresh packages at the package level.