More build instructions

[scheibelp]

Fixes #867

Builds on #697 (merged)

In terms of the requests from 867, this currently implements:

• Generate build instructions for (a) the application along (b) with all its the package dependencies
  • (a) was handled in Features/build instructions #697, this adds (b)
• Generate instructions on where to fetch the source repo for the application (and its dependencies), which branch/release/version to use and what patches, if any, to apply
  • (update July 11): I think I'm doing something more useful, namely downloading all the source using spack mirror create)
  • (update July 14): now this dumps the urls into url-info.txt; moreover, the sources are only downloaded if the --download option is specified
• Dump the entire spack concretizer output
• If possible, dump the spack runtime environment variables used when running the experiment.
  • (there is not an easy way that occurs to me to generate exactly this, but I write a superset of it)
• Write out any conflicts (e.g., CUDA version MFEM doesn't work with - which may be listed in Laghos or MFEM package)
• Write out runtime parameters and environment variables that are set
  • (I now copy the execute_experiment scripts into the output directory)

Other minor changes:

• show-build now creates the output dir if it didn't exist before

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.48%. Comparing base (67bdaeb) to head (7d9a94f).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #890   +/-   ##
========================================
  Coverage    41.48%   41.48%           
========================================
  Files           22       22           
  Lines         1509     1509           
  Branches       213      213           
========================================
  Hits           626      626           
  Misses         838      838           
  Partials        45       45           

Flag	Coverage Δ
unittests	41.48% <ø> (ø)

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