package_hash: add test to ensure that every package in Spack can be unparsed

- [x] add option to canonical source to *not* filter multimethods
- [x] add test to unparse every package in builtin

var/spack/repos/builtin/packages/bash-completion/package.py

@@ -39,7 +39,10 @@ def show_message_to_user(self):
         print('include the following lines in your ~/.bash_profile :')
         print('')
         print('# Use bash-completion, if available')
-        print('[[ $PS1 && -f %s/share/bash-completion/bash_completion ]] && \ ' % prefix)  # NOQA: ignore=E501
+        print(
+            '[[ $PS1 && -f %s/share/bash-completion/bash_completion ]] && \\'
+            % prefix
+        )
         print('    . %s/share/bash-completion/bash_completion' % prefix)
         print('')
         print('=====================================================')