commands: `spack load --list` alias for `spack find --loaded` (#27184)

See #25249 and https://github.com/spack/spack/pull/27159#issuecomment-958163679.
This adds `spack load --list` as an alias for `spack find --loaded`.  The new command is
not as powerful as `spack find --loaded`, as you can't combine it with all the queries or
formats that `spack find` provides.  However, it is more intuitively located in the command
structure in that it appears in the output of `spack load --help`.

The idea here is that people can use `spack load --list`  for simple stuff but fall back to
`spack find --loaded` if they need more.

- add help to `spack load --list` that references `spack find`
- factor some parts of `spack find` out to be called from `spack load`
- add shell tests
- update docs

Co-authored-by: Peter Josef Scheibel <scheibel1@llnl.gov>
Co-authored-by: Richarda Butler <39577672+RikkiButler20@users.noreply.github.com>

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