Runtime dependency on setuptools

[Philippe Normand]

This is a code/packaging issue. Not trivial to reproduce and relevant most likely only for packagers.

Elisa doesn't need to import setuptools at runtime, it's useful only during the build process (when installing elisa and build tarballs).

The plugin_registry imports elisa.core.dist (which imports setuptools). After quick investigation, only 2 functions of dist.py are used by the plugin_registry and they don't rely on setuptools package, which is good. So what we could do is:

- either move those 2 functions (only used by the plugin_registry) to the plugin_registry.py file
- or move them to another (new?) module

[Philippe Normand (philn)]

The Elisa Pigment plugin also imports elisa.core.dist (in pigment_frontend.py) to parse the "mappings" files