Handle no-conda environment, including msgpack

[phyy-nx]

Relevant e.g. for virtualenv builds, using a facility provided python. Uses the msgpack from the bootstrap 'hot' step.

Note in #2319, specifically fb95810, the msgpack3-1-1 reference was removed. The confusion here was that the comment said it was only needed for Python2.7. Turns out that isn't strictly true, as the virtualenv build path uses this hot package.

Someone should make a CI that exercises the virtualenv build path :)

[codecov]

Codecov Report

Merging #2409 (0a87800) into main (f117929) will decrease coverage by 0.07%.
The diff coverage is n/a.

❗ Current head 0a87800 differs from pull request most recent head df5c159. Consider uploading reports for the commit df5c159 to get more accurate results

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- Coverage   78.62%   78.56%   -0.07%     
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  Files         603      603              
  Lines       73644    73644              
  Branches    10005    10005              
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- Hits        57903    57855      -48     
- Misses      13612    13658      +46     
- Partials     2129     2131       +2     

[ndevenish]

Wouldn't this be better handled by e.g. injecting the include path into the build environment at the same point that is responsible for installing the """"""hot"""""" dependency?

[phyy-nx]

I think you are saying add the include directory in bootstrap if use_conda isn't set? That could work too. This PR kinda just puts us back where we were, so it was lower effort :)

FWIW, the old 'hot' mechanism in bootstrap was exactly for this purpose: distributing packages that are frozen due to some functionality being needed for a specific purpose. Perhaps it would be better to call it a 'cold' mechanism :P

If it helps, here's a list of pseudocode steps this build procedure uses (this one is for SACLA):

• bootstrap hot update
• module load python 3.8, openmpi, hdf5. These are custom libraries from the facility.
• python3 -m venv --system-site-packages env. This sets up the virtual environment
• source env/bin/activate
• pip install just all the things
• bootstrap build