commands: add spack resource command to inspect downloadable files

- currently just looks at patches
  - allows you to find out which package applied a patch to a spec

- intended to work with tarballs and resources in the future.

- add tab completion for `spack resource` and subcommands

share/spack/spack-completion.bash

@@ -1041,6 +1041,28 @@ function _spack_repo_rm {
     _spack_repo_remove
 }
 
+function _spack_resource {
+    if $list_options
+    then
+        compgen -W "-h --help" -- "$cur"
+    else
+        compgen -W "list show" -- "$cur"
+    fi
+}
+
+function _spack_resource_list {
+    compgen -W "-h --help --only-hashes" -- "$cur"
+}
+
+function _spack_resource_show {
+    if $list_options
+    then
+        compgen -W "-h --help" -- "$cur"
+    else
+        compgen -W "$(_all_resource_hashes)" -- "$cur"
+    fi
+}
+
 function _spack_restage {
     if $list_options
     then
@@ -1244,6 +1266,10 @@ function _all_packages {
     spack list
 }
 
+function _all_resource_hashes {
+    spack resource list --only-hashes
+}
+
 function _installed_packages {
     spack --color=never find | grep -v "^--"
 }