Code review comment for lp:~bzr/bzr/412930-plugin-path

This patch add the capability to fully control BZR_PLUGIN_PATH by defining magic values
for user, core and site directories and a way to remove them from the search path.

It's compatible with the actual implementation.

This will allow the test/build farm to test:
- the core plugins (the test are currently run with --no-plugins),
- a chosen set of plugins

Thanks in advance to the reviewer :-)