Get rid of is_buildable and build_step flags. Broken by design.

Get rid of is_buildable and build_step flags. Broken by design.

* Auto-setting is_buildable flag in base constructor was pointless
  and broken by design because:
  - is_buildable = check_auto_buildable() used to be called *before*
    constructor of the derivative class could call enforce_* methods.  The
    result of check_auto_buildable() might change after calling enforce_*
    methods (in case check_auto_buildable() use get_buildpath() tests).
  - it isn't used widely. Refactor those a few places.
* Due to above, 'build_step' does not need to be passed to the Buildsystem
  anymore. Remove it from code.
* As a result of is_buidable removal, move warning of
  enforce_in_source_building() to pre_building_step(). It caused unnecessary
  noise when the object was constructed during test. It belongs to
  pre_building_step stage anyway.

Signed-off-by: Modestas Vainius <modestas@vainius.eu>