refactoring: add repo_by_name() and hrepo_by_name() helpers.
disabling/enabling entire repositories.

Because most query filters would require additional checks (as they don't check
repo->disabled, unless they use e.g. the libsolv's dataiterator) the patch goes
for a simplification: unifying the package and repo disables into one bitmap
(sack->excludes).

Goal doesn't have to deal with this, the libsolv solver respects repo->disabled
of course.
tests: cases should not need to reset the sack state.

let the fixtures do that.
py: bindings for disabling/enabling repos.