repoman: add variable.phase check like pkgcheck VariableScopeCheck (bug 608664)

The variable.phase check is inspired by pkgcheck's VariableScopeCheck,
and uses essentially the same PMS data to drive the check.

References:
- https://projects.gentoo.org/pms/7/pms.html#x1-10900011.1
- https://pkgcore.github.io/pkgcheck/_modules/pkgcheck/checks/codingstyle.html#VariableScopeCheck
- https://bugs.gentoo.org/775191

Bug: https://bugs.gentoo.org/608664
Signed-off-by: Zac Medico <zmedico@gentoo.org>
7 files changed