Feature Request - Add support for optional python dependencies through [extra]

[Ryanf55]

Use Case

I use parts of a python package that require the EXTRAS dependencies, and want to specify in my package.xml that I depend on the extras of my dependency.

Example

Pip's networkx has marked an extra dependency on scipy. Because my package uses the kamada-kawai layout feature of networkx, I need to ensure rosdep installs the extra dependencies of networkx such as scipy.

Current dependency in my package.xml, where I must include transitive optional dependencies of python3-networkx

<depend>python3-networkx</depend>
<depend>python3-scipy</depend>

Desired:

<depend>python3-networkx[extra]</depend>

Possible Solution

The rosdep key python3-networkx resolves to the command apt install python3-networkx and the rosdep key python3-networkx[extra] resolves to apt install --install-suggests python3-networkx

$ apt info python3-networkx | grep "Depends\|Recommends\|Suggests"

WARNING: apt does not have a stable CLI interface. Use with caution in scripts.

Depends: python3-pkg-resources, python3-decorator (>= 4.3.0), python3:any
Recommends: python3-numpy (>= 1.15.4), python3-yaml
Suggests: python-networkx-doc, python3-gdal, python3-matplotlib, python3-pydot, python3-pygraphviz, python3-scipy

References

https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies

Originally posted by @Ryanf55 in ros2/ros_network_viz#26 (comment)

[Ryanf55]

Relates to #900