Proposed implementation for #515: Install development dependencies incrementally

[nealstewart]

Description

See [issue 515](https://github.com/conda/conda-lock/issues/515 for background on why this was implemented.

I have implemented this as an optional --update flag to be based to conda-lock install.

This allows for the following use-case, which will support staged Docker builds, e.g. for light-weight testing images:

conda-lock install
conda-lock install --dev --update

I've copied the existing test and there's some potential for deduplication, but I've gone for simplicity. More than happy to refactor the two highly-coupled tests a bit.

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[maresb]

This is exciting! Looks fairly straightforward too. Could you please look into and fix these test failures?

I'm a bit overloaded at the moment, so no promises on timelines for getting this merged, but I'm hoping it will be easy once the CI is green.

[sjdemartini]

I'd love to see this change land—it would be super useful to be able to update an existing env to match a lockfile, rather than deleting/recreating the env as seems to happen now. The latter approach is unsafe for any shared environments (e.g. a JupyterHub kernel env), but conda-lock would be very useful there for updating environments predictably.

Neal, any chance you plan to come back to this and get CI to pass?