Previously, add_py_targets assumed that it is not an error to add an
existing target if that target was previously added using
add_py_targets. This was done to aid testing, but it was weird, error
prone and broke with the latest changes to the caching mechanism. With
this commit:
- it is always an error to add a target that was previously added, which
is much more consistent.
- tests for the configuration mechanism use the newly added
'set_targets_json_location' function to clear the internal caches in
targets.py (and thus all previously added custom targets). As a side
effect, this commit also tests the 'set_targets_json_location'
function itself.
9704edfca7