[recipe-spec] Allow for arbitrary optional dependencies

Proposal:

Have a single top-level requirements section as there is currently with required keys host and run and then optionally allow any other keys to specify named dependencies.

A requirements.build key would then be handled the same as currently but test dependencies would now be included as an optional requirements.test key. The specification of test dependencies under requirements.test would then be consistent with the specification of other dependencies under requirements (e.g. requirements.build rather than build.requirements)

In this way mamba/boa could provide support for the extras_require capability in setuptools/pip which is very widely used, and very useful.

Similar to setuptools, having a special-cased test requirements section could be removed in favour of treating them like any other optional dependency:

To get the benefit of this new capability would require all the requirements to be saved in the package metadata and for mamba to allow users to install any named dependencies in the package metadata.

Currently, it’s very awkward for users to create either a build or test environment as there’s no way to tell mamba to create an environment with those specific deps.

The current recommendation to instead create separate outputs for any optional dependencies is awkward to specify and clutters up package repositories with multiple metadata-only packages for every single build.

[wolfv]

I think the multi-packages / multi-output way would be by far more backward compatible.

I see your point – if we consider having many (empty) packages to be expensive. Let’s let this ferment a bit.

[wolfv]

Regarding moving the test requirements under the requirements.test (instead of test.requires) is something I thought about today and maybe that’s the first step into this direction, actually.