Use recursive include for requirements globbing

[paulbovbel]

Allows using the full range of requirements constraints supported by pip, and gets rid of a bunch of code from catkin_virtualenv. Win-win.

[wkentaro]

Actually, multiple requirements.txt cannot be passed to pip:

% ls req*
req1.txt  req2.txt

% cat req*
PyYAML>=3
PyYAML<=3.1

% pip install -r req1.txt req2.txt
Collecting req2.txt
  Could not find a version that satisfies the requirement req2.txt (from versions: )
No matching distribution found for req2.txt

and combining multiple requirements.txt files is not so easy, I think.
And in most case, it is used with ls req* | xargs -n1 pip install -r, and the order of passed filenames is crucial.

[paulbovbel]

Multiple requirements files can be nominally passed in via multiple -r flags. However, it seems that pip is unable to resolve incompatible (or even intersecting) requirements, although an issue to do proper dependency resolution in pip is in progress (pypa/pip#988 (comment)).

So at the end of the day, this ends up enforcing a system where no requirements are 'merged', and a child package can only inherit it's parent's exact dependencies.

I think that 'merging' requirements intelligently (beyond what I had implemented) is extremely out of scope for this package, and I recognize that it's disingenuous to do it half-assed (hence purging it).

However I would be amenable to an approach that replaces parent dependencies with child dependencies when there's a repeated package key.

[tappan-at-git]

No longer imports wrapt!

[paulbovbel]

sharp eye!

[tappan-at-git]

ditto.

[tappan-at-git]

Hat trick.

[paulbovbel]

I don't think this strategy is viable unless pip can resolve multiple requirements as ticketed. A smarter 'requirement merging' strategy for catkin_virtualenv might be feasible, handling multiple inline constraints.