ros2pkg.api.create_package_environment: handle exports

[tim-fan]

Feature request

Feature description

In brief, I'm requesting package.xml.em be updated to allow setting of export tags, and create_package_environment be updated to pass export tags from the package description to the template.

Background: I'm using ros2pkg.api to generate a package. Looking at create.py#L148, it looks as if I should be able to list package exports in the catkin_pkg.package.Package specification, and have them listed in the package.xml generated by create_package_environment. However it appears that create_package_environment ignores the package export field, and the template only sets the build_type export tag.

I found it slightly confusing that create.py#L148 sets this export field, even though it is completely ignored by create_package_environment

Implementation considerations

[audrow]

Hi @tim-fan, you're right: it is weird that the export field is set and then not used by create_package_environment. Feel free to make a pull request for this - you can @ me if you need any help or need a review.

[audrow]

Closed with #628.