Currently, the lint target does a sequential: pep8, pylint, pylint on
the QA sources. hlint is run only when explicitly requested.

This patch reorganises the lint target by:

- splitting the current lint target into separate pylint (slow),
  pylint-qa (fast), pep8 (fast) targets
- making lint itself depend on the above plus hlint
- both pep8 and hlint are optional, based on whether the said binaries
  were detected at configure time

make -j is slightly faster after this patch (1m18s → 1m7s).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>