building fontconfig with ESM on xenial & non-xenial

When building snapd on xenial, instead of fetching fontconfig from xenial-security, it's probably best to fetch it with `apt-get source fontconfig` because ubuntu-esm repositories might be active in the build environment, and there could be fontconfig update published in ESM.

I'm not sure how to get that working when/if buildig snapd on non-xenial.

I wonder if we should copy any fontconfig packages from those locations to snappy-edge PPA.

Start by tracing how this repository builds snapd on xenial and non-xenial systems, focusing on fontconfig source retrieval and active ubuntu-esm repositories. Determine whether apt-get source fontconfig should replace xenial-security fetching and whether packages should be copied to the snappy-edge PPA; verify the result for both release classes.

Written by the indexing model from the issue text.