When a user specifies a --only/--except option in their command-line,
and it turns-out to be unused, Packer signals it with a Warning
diagnostic.
This diagnostic, in previous versions of Packer, would only state that
one option did not match anything, without stating which option did not
match a build or post-processor.
This commit tracks the usage of each option so we can report later which
of those options was not used in this warning diagnostic.
Lucas Bajolet
1a0be5ec6f