Merged 'purge' command with 'clean' and deleted 'purge' (#4970)

* Merged 'purge' command with 'clean'. Deleted 'purge'. fixes #2942

'spack purge' has been merged with 'spack clean'. Documentation has been
updated accordingly. The 'clean' and 'purge' behavior are not mutually
exclusive, and they log brief information to tty while they go.

* Fixed a wrong reference to spack clean in the docs

* Added tests for 'spack clean'. Updated bash completion.

share/spack/spack-completion.bash

@@ -175,7 +175,8 @@ function _spack_checksum {
 function _spack_clean {
     if $list_options
     then
-        compgen -W "-h --help" -- "$cur"
+        compgen -W "-h --help -s --stage -d --downloads
+                   -m --misc-cache -a --all" -- "$cur"
     else
         compgen -W "$(_all_packages)" -- "$cur"
     fi
@@ -598,11 +599,6 @@ function _spack_providers {
     fi
 }
 
-function _spack_purge {
-    compgen -W "-h --help -s --stage -d --downloads
-                -m --misc-cache -a --all" -- "$cur"
-}
-
 function _spack_python {
     if $list_options
     then