Fix support for parsing VCS uris with an @ sign

[mgmarino]

As of #310, this fails to output the correct reference.

Example:

>>> from requirementslib import __version__
>>> __version__
'1.6.4'
>>> from requirementslib.models.requirements import Line
>>> Line("-e git+ssh://git@github.com/mycomp/our_repo.git@release/v318#egg=our_package")
<Line (editable=True, name=our_package, path=None, uri=git+ssh://git@github.com/mycomp/our_repo.git, extras=(), markers=None, vcs=git, specifier=None, pyproject=None, pyproject_requires=None, pyproject_backend=None, ireq=None)>

>>> from requirementslib import __version__
>>> __version__
'1.6.6'
>>> from requirementslib.models.requirements import Line
>>> Line("-e git+ssh://git@github.com/mycomp/our_repo.git@release/v318#egg=our_package")
<Line (editable=True, name=our_package, path=None, uri=git+ssh://git@github.com/mycomp/our_repo.git@release/v318, extras=(), markers=None, vcs=git, specifier=None, pyproject=None, pyproject_requires=None, pyproject_backend=None, ireq=None)>

Related to pypa/pipenv#5179