Allow loose specification of explicit url

[niosus]

Currently, when specifying a specific Url for a package it has to be specified in full. Allow for more freedom here. For example. Currently one has to do the following:

  <export>
    <git_url target="foo" url="https://github.com/USER/foo.git"/>
  </export>

It would be nice to not duplicate the name of the package if applicable:

  <export>
    <git_url target="foo" url="https://github.com/USER"/>
  </export>