add environment variable info to meta data

[bcumming]

Add information about the environment variable changes to be applied when starting a view to the $mount/meta/env.json file in the generated image.

• replace the add-compiler-paths.py script with envvars.py
• the activate.sh script is not longer used

Add two new views: spack and modules.

• the modules view replaces the uenv modules use command
• these link to /dev/null, so that activating them with older versions of uenv is a noop.

Additional fixes:

• support workflows where no Spack repos are provided
  • this was broken, because on Alps there is always at least one external repo.
• add error handling for the initial Spack bootstrap
  • this was not failing if there was an error