app-admin/rex: Update -9999 ebuild to current status.

app-admin/rex: Update -9999 ebuild to current status.

- Migrate to EAPI7
- Switch to branch 'master'
- Update DESCRIPTION
- Add new dependencies
- Add patching to dzil live code that eliminates Plugin::Test::
  dependencies that are not likely to be useful for consumers building
  code on Gentoo for their own purposes, especially as these massively
  complicate the amount of work to satisfy dependencies.
- Add glue to populate portage ${HOME} to satisfy NextVersions' need to
  have [%User] configured without erroring.

Some of these changes are borrowed from FErki's PR to bump to 1.9x,
but the changes are done in-band for clarity, and the dependency logic
is re-evaluated whosale from the start, to eliminate dependencies that
aren't needed.

Bug: https://bugs.gentoo.org/705190
Bug: https://github.com/gentoo/gentoo/pull/14313
Package-Manager: Portage-2.3.97, Repoman-2.3.22
Signed-off-by: Kent Fredric <kentnl@gentoo.org>