CEP request: Standardize http/filesystem channel structure

Current practice are based on de facto implementations in conda and conda-index. For clarity, this should be standardized.

Roughly, current setup is:

- `path/or/url/to/channel-name/subdir/repodata.json`
- `path/or/url/to/channel-name/subdir/package_name-version-build_string.tar.bz2`

Some parts of `conda` require at least `/repodata.json` to always be present.

In some cases, channel name is empty because the host directly exposes the subdirs in the root (e.g. `http://my-custom-channel.org/noarch/repodata.json`). This seems to be unintended and also affects how `channel::package` specs work.

We don't have clear rules for allowed characters or case sensitivity, either.

I wonder if this should be standardized in the context of a HTTP API explicitly (e.g. document expected result of `GET` requests), like the Open Container specification.