Plugins: Update entrypoint group for pip origin

• Plugins: Update entrypoint group for pip origin

  As discussed in https://mail.gnome.org/archives/buildstream-list/2020-January/msg00001.html, separate the entry point groups for elements and sources. Previously, both used buildstream.plugins which meant that it wasn't possible to have an element and a source plugin with the same name, when using pip origin.

  This a breaking change for all plugins, but will only affect projects using the pip plugin origin.

• NEWS: Announce new entry point groups for plugins

• doc/sample_plugin: Update name of entry point group

  Update sample plugin documentation based on the recent change to entry point group for plugins.

  While I'm here, also remove the unnecessary dependency on setuptools from the sample plugin.