Currently, environments created from manifest files with relative includes result in broken
references to config files.
This PR modifies `spack env create` to create local copies in the new environment of any local
config files from relative paths in the environment manifest passed as an init file.
This PR does not change the behavior if the include is an absolute path or if the include is from
a relative path outside the environment directory, but it does warn about missing relative includes if
they are inside the environment directory.
Includes regression test and short blurb in docs.