When running a packer command on an HCL2 template, depending on whether
or not there are required_plugin blocks defined, Packer may need to
discover and register a plugin's components multiple times.
This is the behaviour ever since those blocks were introduced to Packer,
but given we are doing the operation multiple times, this is suboptimal.
This commit changes the way things works by first doing the restricted
discovery of plugins (as dictated by required_plugins), then proceeding
to the global discovery, with the change that subsequent component
discoveries will not have precedence over those pre-discovered anymore.
This allows us to invert the call order of both discovery phases safely,
and maintains the constraints described in the templates.
Lucas Bajolet
3e3f56ad98