Summary:
Move top-level Python packages `mercurial`, `hgext` and `hgdemandimport` to
a new top-level package `edenscm`. This allows the Python packages provided by
the upstream Mercurial to be installed side-by-side.
To maintain compatibility, `edenscm/` gets added to `sys.path` in
`mercurial/__init__.py`.
Reviewed By: phillco, ikostia
Differential Revision: D13853115
fbshipit-source-id: b296b0673dc54c61ef6a591ebc687057ff53b22e
