[v3 Downloader] RFC: installing cogs/shared libs from PyPI + cog discovery

# Feature request

#### Select the type of feature you are requesting:
- [x] Core (Downloader)

#### Describe your requested feature
Since redbot can be installed directly using `pip`, and will be usable without `git` once discord.py v1.0+ hits PyPI, I think that Red should support cogs or cog collections installed as python packages.

Fine-grained management via Downloader (beyond just calling `[p]pipinstall`) would be nice to have, but using [setuptools plugin discovery](https://packaging.python.org/guides/creating-and-discovering-plugins/#using-package-metadata) ([docs](https://setuptools.readthedocs.io/en/latest/setuptools.html#dynamic-discovery-of-services-and-plugins)) is a good first step.

#### Benefits:
- Dependency management and updates are already handled
- Supports variants/dependencies for specific OSes or via [setuptools "extras"](https://setuptools.readthedocs.io/en/latest/setuptools.html#id16)
- Versioning, stability level and other metadata is well-defined
- Cog installation can be done in one step using `pip install`, without adding any repos
- Cogs can support pre-compiled native code packaged as a wheel
- Release pipelines are easier to set up

#### Current issues:
1. Package discovery for normal packages is independent of repo contents
- packages not installed will not appear during discovery
- sources of cog list/metadata could be a "base" package, or cogs.red.
- base package would have to be updated when cogs are added/removed, description changes, etc.
- cogs.red would have to perform discovery (or just rescan when a webhook fires)
2. PyPI has no tag/classifier for Red ([yet](https://github.com/pypa/warehouse/issues?q=is%3Aissue+is%3Aopen+label%3A%22classifier+request%22))
3. Hybrid repositories
- setup.py reading from info.json?
- repo manager should tolerate namespace packages in git repos
- installation may need to parse requirements.txt instead of JSON requirements array