Support PEP 338 invocation of rosdep2 module

[cottsay]

This package has a well-established entry point. It would be great to gain access to that entry point in a way that allows use of a specific interpreter and bypasses PATH.

[codecov-commenter]

Codecov Report

Merging #862 (89d621e) into master (6b05600) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #862      +/-   ##
==========================================
- Coverage   75.03%   74.97%   -0.07%     
==========================================
  Files          42       43       +1     
  Lines        3333     3336       +3     
==========================================
  Hits         2501     2501              
- Misses        832      835       +3     

Impacted Files	Coverage Δ
src/rosdep2/__main__.py	0.00% <0.00%> (ø)

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[nuclearsandwich]

So I understand correctly, is goal of this PR is to enable the use of

python3 -m rosdep2 update so that it's equivalent (assuming your path is rational and the rosdep bin and python3 bin are from the same environment) to rosdep update

I don't doubt that this is the way to go about it but I'm not quite able to connect the dots myself between the runpy.run_module signature in https://peps.python.org/pep-0338/ and the addition of this file. So I'd appreciate some support in understanding how this works.

[cottsay]

I'm not quite able to connect the dots myself between the runpy.run_module signature in https://peps.python.org/pep-0338/ and the addition of this file.

From the abstract:

This PEP defines semantics for executing any Python module as a script, either with the -m command line switch, or by invoking it via runpy.run_module(modulename).

I referenced this pep because it describes the "desired behavior" of how I'd like to invoke this module (both via -m and runpy.run_module). Unfortunately, I'm a little lost on the details of how the connection to __main__ is made, but the non-pep documentation is pretty clear, and also references pep 338: https://docs.python.org/3/using/cmdline.html#cmdoption-m

[nuclearsandwich]

I forgot to circle back after my questions were answered.

Looks like an entirely useful change. Thanks!