colcon dependency management does not support `<doc_depend>`

[guillaumeautran]

Based on the code, colcon-ros does not seem to support the <doc_depend> type of dependencies. These dependencies behave similarly as <build_depend> but are more proper as <doc_depend> when related to doc generation. Thinks like Doxygen or Sphinx (or rosdoc2 in my case of interest).

[mikepurvis]

Mentioned briefly in #128 (comment), with the context of wanting an accurate dependency picture for the purposes of running documentation builds from colcon (also ros-infrastructure/rosdoc2#15).

I think the easiest first pass would just be to add a stanza to this block which propagates the doc_depends from catkin_pkg into desc.dependencies['doc']:

colcon-ros/colcon_ros/package_identification/ros.py

Lines 86 to 106 in eb0b932

	for d in pkg.build_depends + pkg.buildtool_depends:
	assert d.evaluated_condition is not None
	if d.evaluated_condition:
	desc.dependencies['build'].add(DependencyDescriptor(
	d.name, metadata=_create_metadata(d)))
	for d in (
	pkg.build_export_depends +
	pkg.buildtool_export_depends +
	pkg.exec_depends
	):
	assert d.evaluated_condition is not None
	if d.evaluated_condition:
	desc.dependencies['run'].add(DependencyDescriptor(
	d.name, metadata=_create_metadata(d)))
	for d in pkg.test_depends:
	assert d.evaluated_condition is not None
	if d.evaluated_condition:
	desc.dependencies['test'].add(DependencyDescriptor(
	d.name, metadata=_create_metadata(d)))

Though of course to actually do something meaningful with these, the relevant calls to PackageDescriptor.get_recursive_dependencies would need to specify the correct categories:

https://github.com/colcon/colcon-core/blob/a6aef1515b946f5af47b16d55b4f9ffaaaa27126/colcon_core/package_descriptor.py#L95-L96