Config prefer upstream (#21487)

This allows for quickly configuring a spack install/env to use upstream packages by default. This is particularly important when upstreaming from a set of officially supported spack installs on a production cluster. By configuring such that package preferences match the upstream, you ensure maximal reuse of existing package installations.

share/spack/spack-completion.bash

@@ -587,7 +587,7 @@ _spack_config() {
     then
         SPACK_COMPREPLY="-h --help --scope"
     else
-        SPACK_COMPREPLY="get blame edit list add remove rm update revert"
+        SPACK_COMPREPLY="get blame edit list add prefer-upstream remove rm update revert"
     fi
 }
 
@@ -631,6 +631,10 @@ _spack_config_add() {
     fi
 }
 
+_spack_config_prefer_upstream() {
+    SPACK_COMPREPLY="-h --help --local"
+}
+
 _spack_config_remove() {
     if $list_options
     then