Remove plugins which have been promoted to `buildstream-plugins`

Similar to #45 (closed) with the docker source (which has also been moved to buildstream-plugins), we should consider removing the following plugins from this repo which have been promoted to buildstream-plugins:

Sources:

• cargo
• pip (this was actually migrated from buildstream and not from here, not sure why there is a duplicate plugin here, it may contain some features not supported from the original pip source)

Elements:

• meson
• cmake
• make
• pip
• distutils (renamed to setuptools in buildstream-plugins)