Bugfix: allow missing modules if they are blacklisted (#13540)

`spack module loads` and `spack module find` previously failed if any upstream modules were missing.  This prevented it from being used with upstreams (or, really, any spack instance) that blacklisted modules.

This PR makes module finding is now more lenient (especially for blacklisted modules).

- `spack module find` now does not report an error if the spec is blacklisted
  - instead, it prints a single warning if any modules will be omitted from the loads file
  - It comments the missing modules out of the loads file so the user can see what's missing
  - Debug messages are also printed so users can check this with `spack -d...`

- also added tests for new functionality