Support for ".conda" packages

Apparently, https://repo.anaconda.com/pkgs/main now supports a speedier version of the tarballs dubbed .conda packages (https://www.anaconda.com/how-we-made-conda-faster-4-7/). It is curious that the index.html generated by conda index does not show them, but you can try and download any of the packages by replacing the .tar.bz2 extension by .conda, and it works.

This does not work (yet) for our mirrored packages or for our own channel. We could potentially speed-up further our builds if we dropped ".tar.bz2" support and adopted the new format. However nice, it will not come without work.

• Our mirroring tool conda-mirror needs to support .conda packages. I opened an issue on that repo. Let's see if the main author gets interested: https://github.com/Valassis-Digital-Media/conda-mirror/issues/89. If that does not happen, we can either change the project ourselves to fix this (and submit a patch), or mirror the defaults repo via curl/wget/conda-index ourselves.
• We need to modify our build instructions and CI setup to produce and upload .conda packages as well (conda config --set conda_build.pkg_format 2).

The good news is that both things are independent, so we can go on with our stuff and wait for a solution for the first.