Add per package CPM_DOWNLOAD controls

[robertmaynard]

Introduces support for CPM_<PACKAGE>_DOWNLOAD variable ( and env ) which allows finer grained control.

A common workflow pattern we have run into is wanting to set CPM_DOWNLOAD_ALL to on for a middle package in a CPM dependency chain.

Root
  -> CPM project A
       -> CPM project B 
       -> CPM project C

This would allow us to state that project A needs to be downloaded but project B or C can be found on disk.

[iboB]

Thanks for the PR

[OlivierLDff]

Interesting this like the variant I thought here #280 (but still didn't take time to implement)

[iboB]

I can't think of how this can break anything. Looks good to me.

@TheLartians what do you think?

Also, this makes is a good use case for an integration test, I'll try do add one for this over the weekend

[TheLartians]

Agreed, this looks good! Definitely worth it to add an integration test, so we don't accidentally break it in the future.

[TheLartians]

@iboB are you still planning to write some integration tests for this feature?

[iboB]

Yes. At least a very bare-bones one. I'm a bit short on time, though. I think if we're satisfied with this, we can merge and I can add tests later

[TheLartians]

Alright, thanks @robertmaynard for the feature!