"prequ edit" command

[suutari-ai]

Implement prequ edit PACKAGE command which can be used to turn a requirement to editable.

The command should:

1. Fetch the sources of the package to a directory like deps/PACKAGE or to a user specified directory (with --dir DIRECTORY or via deps_dir configuration variable). If there already is sources in the directory, don't override them.
   • Note: This tries to create a VCS checkout when possible, e.g. if there is a Git repository found via package info, it should be cloned and checked out to the currently pinned version.
2. Make the package temporarily editable in the current virtualenv
   • If the package is currently installed in the current virtualenv (which should be active), reinstall it as editable from the directory specified in 1.
   • Store the info that the package is editable into a .prequ.state file so that prequ sync knows to leave it so.

The editable status can be reverted back with prequ unedit PACKAGE. This won't remove the source directory, but it reinstalls the package to make it non-editable in the virtualenv again. It also updates .prequ.state.