Users requested an option to filter between local/upstream results in `spack find` output.
```
# default behavior, same as without --install-tree argument
$ spack find --install-tree all
# show only local results
$ spack find --install-tree local
# show results from all upstreams
$ spack find --install-tree upstream
# show results from a particular upstream or the local install_tree
$ spack find --install-tree /path/to/install/tree/root
```
---------
Co-authored-by: becker33 <becker33@users.noreply.github.com>
7e906ced75