Add ignore mechanisms for discovery via env, package.xml, and XML marker

[Maik13579]

This adds three opt-out mechanisms for discovery.

Packages can now be ignored through the YASMIN_DISCOVERY_IGNORE_PACKAGES environment variable, through a flag in package.xml, or per state machine XML file with a special ignore comment on the first non-empty line.

The env-based ignore list is also included in the cache state so discovery results are invalidated correctly when the ignore list changes.

export YASMIN_DISCOVERY_IGNORE_PACKAGES="pkg_a,pkg_b"

<export>
  <yasmin_plugins_manager ignore="true"/>
</export>

<!-- YASMIN_IGNORE_DISCOVERY -->
<state_machine>
  ...
</state_machine>

I added the ignore tag to yasmin_factory so that the test state machines are ignored.