Added mamba support

[GabrielDougherty]

This implements the ability to use mamba, in accordance with our design discussion.

Design considerations

I commented out the call to _update_conda. I found it confusing that we specify the version of conda in the build file, but then ask conda to update itself. This seems to go against the "Bazel way" (if I can call it that) of specifying the exact versions for every input to the build process. Can I remove the _update_conda function?

Testing

I manually tested this by running the test in tests/ with the following modifications:

• conda then mamba w/o clean -> success
• mamba then conda w/o clean -> success
• conda then mamaba w/ clean -> success
• mamba then conda w/ clean -> success

To elaborate on what these bullet points mean, for the first bullet point I would do the following:

cd tests/
bazel clean --expunge
# edit WORKSPACE to set use_mamba = True and clean = False
nvim WORKSPACE
bazel test test
# edit WORKSPACE to set use_mamba = False and clean = False
nvim WORKSPACE
bazel test test

I would repeat this process, manually changing the value of use_mamba and clean for each bullet point. In between each bullet point, I run bazel clean --expunge. Now I would like to make an automated version of these tests, but the only idea I have for that is to make a bunch of directories under tests/ with their own WORKSPACE and BUILD files and change the use_mamba and clean flags to the matrix of combinations I specified above. Then there would be a test script that goes to each directory and runs bazel test //.... The problem is that that approach would duplicate a lot of files, just to change two flags. Do you have any better idea for structuring the tests?

TODO

Before merging this PR, I would like to:

• Add documentation for the new flags
• Add tests

[spietras]

Great job so far! Added more suggestions, this time for rather smaller things

[spietras]

Please rebase after #21

[spietras]

Wonderful, we are almost there, only final touches now. Ping me when you think it's ready to merge.

[GabrielDougherty]

@spietras ready to merge, thanks for your prompt code reviews!

[spietras]

Awesome, great job 💯

Thanks for contributing to rules_conda 💚