uberenv updates to support dev with python in spack envs

[cyrush]

Updates that adjust uberenv spack env setup so that folks can develop using python modules installed to a spack view.

Addresses most challenges outlined #120.

May still need some cleanup, so comments welcome.

Details:

• Adds a new step that patches host configs post build to use the paths in the spack view.

Uses patterns from .uberenv_config.json spack_host_config_patches` entry to drive what is patched:

Example input:

  "spack_host_config_patches":
 {
   "PYTHON_EXECUTABLE": "view/python*/bin/python3",
   "SPHINX_EXECUTABLE": "view/python*/bin/sphinx-build"
   }

Creates a -patchd.cmake file in the prefix dir (does not modify original host config)

• adds --spack-skip-externals option

This is mostly driven by the limitations of using an external python, but is generally useful for cases where system libs undermine you.

• symlink the default view of the spack env into {prefix}/spack_view for easy access.

- patch spack.yaml to create the view dir under spack_env, instead of a hidden file deeper in the dir hierarchy

This should only happen in cases where spack.yaml lacks view customization.

• fixes symlinking host config is broken with envs + extra spec constraints #123

[cyrush]

Looks like i need to fix the view dir patching logic.

[cyrush]

part of a fix for #123

[cyrush]

part of a fix for #123

[cyrush]

I removed the logic that tried to present a better default view by hacking yaml, and instead opt-ed to symlink whatever the view directory is under the prefix. This should be more robust and work with any view setup even if it was customized in an input spack.yaml

[cyrush]

I need to test why we can't inspect and symlink the view for the --install case.

==> Updating view at /home/runner/work/uberenv/uberenv/.ci/test-project/uberenv_libs/spack_env/.spack-env/view
[exe: /home/runner/work/uberenv/uberenv/.ci/test-project/uberenv_libs/spack/bin/spack -D /home/runner/work/uberenv/uberenv/.ci/test-project/uberenv_libs/spack_env python -c 'env = spack.environment.active_environment();print(env.views["default"].get_projection_for_spec(env.matching_spec("'magictestlib_cached@1.0.0%gcc ^hdf5@1.14.0'")))']
[failed to find spack view info]

[cyrush]

@white238 @kennyweiss @chapman39

This branch has been tested and is now in use for both conduit and ascent CI.
I would like to find a path to get it into uberenv main