hghave: move feature checking into hghave.py
Upcoming patches will kill hghave (the script - not hghave.py) and
will move to a model where requirements checking is performed as
a function call.
Start diminishing the utility of hghave by moving some code to
hghave.py.
tests: move '#require bzr' into .t files
The bzr tests are the only .t tests calling hghave (aside from the tests
testing hghave itself). This pattern is a one-off and prevents
desired refactors to how requirements checking is integrated into tests.
Use the convention used everywhere else.