cargo: add links = "python"

[indygreg]

PyOxidizer has crates depending on pyo3 that would like to access
the pyo3 crate configuration. (This use case isn't unique to
PyOxidizer.)

Cargo has a facility for enabling the build scripts of dependent
crates to access exported variables via DEP_ environment
variables. However, this only works if the exporting crate defines a
links key in its Cargo manifest. See
https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key.

While pyo3's build script doesn't yet export the variables that
PyOxidizer will need, a prerequisite to making this work is adding
the links key. Since this change could introduce unintended
side-effects, it warrants being made in its own commit, which is
why we're making this change outside of #1793.

I think this change should be mostly safe: the links key is
effectively metadata advertising that a crate links against a named
library. The only side-effects setting it has is to enable the
aforementioned DEP_ environment variables in build scripts and
enforcing a limitation that only a single crate may link against the
same native library. I believe the only potential for this change
to cause problems is if there are multiple crates with links = "python" entries. I'm not aware of any other crates that advertise
links = "python": even python3-sys / cpython use links = "python3" so this change should not prevent dual use of pyo3 and
cpython in the same build.

[davidhewitt]

FYI I'm sitting on merging this just because I want to understand whether it's really necessary. So I'd like to see the result of indygreg/PyOxidizer#433 (comment)

TBH it's probably a good idea to add this key anyway, however no harm in waiting. I'll make sure this gets merged before 0.15 if deemed relevant.

[birkenfeld]

Wasn't it discussed to add links somewhere else too (to avoid multiple pyo3s in the dep graph)?

[davidhewitt]

Yes, though I can't remember exactly where. That's what I was implicitly referring to when I said "it's probably a good idea to add this key anyway" - I should have been clearer 🙃

[davidhewitt]

Ok, shall we go ahead and merge this then? Looks like it needs a rebase; there's a CHANGELOG merge conflict and also some kind of glitch with CI which probably will get fixed with a rebase.

[indygreg]

I rebased to resolve the merge conflict.

[davidhewitt]

Thanks!