* Add integration tests for ansible-doc.
* Enable tests that now pass
* Cleanup processing of plugin docs
* Mostly separate the steps of processing plugin docs
1) Acquire source data
2) Transform and calculate additonal data
3) Format data for output
4) Output data
format_plugin_doc() is still mixing transformation and formatting but
that should be fixed in a devel-only change
* Raise exceptions in _get_plugin_doc() on errors.
* Remove check to exclude on blacklisted extensions. We already request
only .py files
* If there is no DOCUMENTATION entry in the plugin, raise an exception
from _get_plugin_doc(). Everywhere we use _get_plugin_doc(), this is
treated as an error
* If there is no ANSIBLE_METADATA raise an exception as well as
displaying of docs assumes that this has been set.
* If there is neither DOCUMENTATION nor ANSIBLE_METADATA, warn about the
lack of METADATA and error on the lack of DOCUMENTATION. Lack of
DOCUMENTATION is more important so it is what the user should see.
* Add a few special cases for backwards compat. These should probably
be made errors in 2.10:
* no docs but has metadata shows no documentation rather than an error
* empty plugin file shows no doumentation rather than an error
* Simplify backwards compatibility logic.
