Support Spack.yaml files split in several files

[adrienbernede]

We moved RADIUSS Spack Configs to Spack environments.
Some RADIUSS Spack Configs users would prefer to have a direct access to the equivalent packages.yaml and compilers.yaml.

When setting spack.yaml to include those files through a relative path, using that environment in uberenv fails because at environment creation, the includes are not copied over at the destination path.

uberenv/uberenv.py

Line 907 in 205672b

spack_create_cmd = "{0} env create -d {1} {2}".format(self.spack_exe(use_spack_env=False),

I'd like to allow that behavior by first copying the content of the directory containing the original spack.yaml file and then run the spack create directly in the destination directory (although I'm not sure whether that would still be necessary).

@white238 @chapman39 what are your thoughts about this ? Did I miss a requirement ?

[adrienbernede]

Demo PR coming