commands: add spack license update-copyright-year

This adds a new subcommand to `spack license` that automatically updates
the copyright year in files that should have a license header.

- [x] add `spack license update-copyright-year` command
- [x] add test

share/spack/spack-completion.bash

@@ -1042,9 +1042,9 @@ _spack_install() {
 _spack_license() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help"
+        SPACK_COMPREPLY="-h --help --root"
     else
-        SPACK_COMPREPLY="list-files verify"
+        SPACK_COMPREPLY="list-files verify update-copyright-year"
     fi
 }
 
@@ -1053,7 +1053,11 @@ _spack_license_list_files() {
 }
 
 _spack_license_verify() {
-    SPACK_COMPREPLY="-h --help --root"
+    SPACK_COMPREPLY="-h --help"
+}
+
+_spack_license_update_copyright_year() {
+    SPACK_COMPREPLY="-h --help"
 }
 
 _spack_list() {