added code to support multi_deps for extensions only

[mboisson]

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This can save valuable time when only the extensions require multi_deps. For example, this could be used to install Matlab, or TensorRT with python bindings, without re-installing those for each version of python.

Ideally, in this case, I would want to skip extracting the source, and keep the version that was built in the previous iteration, but I could not figure out the right combination of options for that, so I kept extracting the source here.

I tested this with the TensorRT recipe, which installs python wheels that are part of the extracted binary.

I would like @bartoldeman and @boegel's comments on this PR.

[akesandgren]

Looks sane but I'll defer to @boegel

[mboisson]

Will this be included in 4.2.1 @boegel ?

[boegel]

@mboisson Tests are missing, so this is not ready to go in yet imho...

I haven't had a chance to closely follow up on this, but i don't think this should block the 4.2.1 release (which we hope to push out any day now).

I'm moving this to the milestone for the release after 4.2.1...

[mboisson]

@boegel, what kind of test would you like to see ? I'm not sure how to test this.

[boegel]

@mboisson To get this covered in the tests, see how we test the use of multi_deps?

[mboisson]

I'm sorry, I don't have time to work on tests at the moment.