new command: spack maintainers queries package maintainers

- We don't currently make enough use of the maintainers field on
  packages, though we could use it to assign reviews.

- add a command that allows maintainers to be queried
  - can ask who is maintaining a package or packages
  - can ask what packages users are maintaining
  - can list all maintained or unmaintained packages

- add tests for the command

share/spack/spack-completion.bash

@@ -700,6 +700,16 @@ function _spack_log_parse {
     fi
 }
 
+function _spack_maintainers {
+    if $list_options
+    then
+        compgen -W "-h --help -a --all --maintained --unmaintained
+                    --by-user" -- "$cur"
+    else
+        compgen -W "$(_all_packages)" -- "$cur"
+    fi
+}
+
 function _spack_mirror {
     if $list_options
     then