Expand relative dev paths in environment files (#22045)

* Rewrite relative dev_spec paths internally to absolute paths in case of relocation of the environment file

* Test relative paths for dev_path in environments

* Add a --keep-relative flag to spack env create

This ensures that relative paths of develop paths are not expanded to
absolute paths when initializing the environment in a different location
from the spack.yaml init file.

share/spack/spack-completion.bash

@@ -793,7 +793,7 @@ _spack_env_deactivate() {
 _spack_env_create() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -d --dir --without-view --with-view"
+        SPACK_COMPREPLY="-h --help -d --dir --keep-relative --without-view --with-view"
     else
         _environments
     fi