[*] just allow plugins from the sources to be found, so the tests only test those :)
[*] plugins_dir_paths now uses the data dirs instead of the config dirs.
[-] this file should not be needed anymore. use python3 setup.py install --prefix <foo>.
[*] in fact, we should also put this source dir before the rest, so also the tests use this pbt package and not one found somewhere else in the PYTHONPATH.
[*] ctx.path_to_plugin_file() does not return a non-existant file anymore. set a basic environment so it finds the local plugins.
[+] path_to_plugin_file() now does return at least the user's local plugin dir, so they can be downloaded there.
[*] use the xdg_local_dirs for check results, setp 1.
[*] fix all test cases with the new ctx.plugins_dir_paths semantic.
[*] renamed bootstrapping script.