modules: use new `module` function instead of `get_module_cmd` (#8570)

Use new `module` function instead of `get_module_cmd`

Previously, Spack relied on either examining the bash `module()` function or using the `which` command to find the underlying executable for modules. More complicated module systems do not allow for the sort of simple analysis we were doing (see #6451).

Spack now uses the `module` function directly and copies environment changes from the resulting subprocess back into Spack. This should provide a future-proof implementation for changes to the logic underlying the module system on various HPC systems.