Support gitlab private pypi packages

We should support an environment variable PYPI_GITLAB_URL which would be a list of URLs (separated by a whitespace) that should be tried to find the dependencies.

These responses are different from Pypi's, the schema is described here. We should have a dedicated function parsing these. The list of packages is finite there so this search could be done beforehand to fill a dictionary with the same structure as Pypi's versions.

When looking for a package's versions, first see if it's provided by one of the PYPI_GITLAB_URL. Then fallback to pypi.