csh: don't require SPACK_ROOT for sourcing setup-env.csh

[tgamblin]

setup-env.sh and setup-env.fish know how to find themselves in bash, zsh, dash, and fish and how to set SPACK_ROOT automatically. setup-env.csh doesn't, and we still have to tell people to set SPACK_ROOT in this one case for csh, which can make initialization instructions confusing.

This uses the same trick we use for dash for csh and tcsh, and it adds some rudimentary testing for setup-env.csh to ensure it keeps working. It turns out we were broken for regular csh because it does not supporrt [, so this has been refactored as well.

New output looks like this:

(spackbook):~$ src/spack/bin/spack env activate .
==> `spack env activate` works best with spack's shell support.
  
  To set up shell support, run the command below for your shell.
  
  For bash/zsh/sh:
    . /Users/gamblin2/src/spack/share/spack/setup-env.sh
  
  For csh/tcsh:
    source /Users/gamblin2/src/spack/share/spack/setup-env.csh
  
  For fish:
    source /Users/gamblin2/src/spack/share/spack/setup-env.fish
  
  Or, if you want to use `spack env activate` without shell
  support, you can run one of these:
  
      eval `spack env activate --sh   .`  # bash/zsh/sh
      eval `spack env activate --csh  .`  # csh/tcsh
      eval `spack env activate --fish .`  # fish

(spackbook):~$ 

• Add logic to setup-env.csh to automatically find the script and set SPACK_ROOT
• Add simple tests for setup-env.csh
• Refactor use of [ out of setup-env.csh so it works with plain csh
• Remove SPACK_ROOT from csh/tcsh instructions
• Add fish to shell instructions
• make output consistent across commands

[tldahlgren]

There's so much in common between the activate and deactivate messages that it seems like this could be factored out and passed the subcommand to reduce redundancy and ensure consistency.

[tldahlgren]

LGTM. Seems to work fine with tcsh and csh. My only suggestion is to refactor the message generation for activate, deactivate, load and unload since there is a significant amount of overlap between the messages.

Also, did you want to update the documentation here or in a separate PR for:

• getting_started.rst
• module_file_support.rst
• packaging_guide.rst

[tgamblin]

@tldahlgren: ok this is consolidated. Please rebase if you merge it.

Updated docs are in a separate PR, #19486.

[tldahlgren]

LGTM