Fixes relative to RPATH handfling

[dnicolodi]

Builds on top of #783 and replaces #724

[dnicolodi]

This should fix RPATH handling for good.

The only thing not included is automatic translation of the $ORIGIN anchor in install_rpath arguments to @loader_path on macOS. It would not be hard to implement, however, it would be a deviation from what Meson implements. I am not sure it is a good idea to implement it.

There is one case in which this may break projects that work now: when libraries or modules require setting an RPATH to dynamically link to a library installed in the Python install path (with something like install_dir: py.get_install_dir() / 'package') and that work now for how because meson-python does not remove build RPATH entries added by Meson and the source layout matches the install layout. This was the case for example for SciPy 1.15, see #724 (comment) and previous discussion. The comments there indicated that, for what SciPy is concerned, it should be fine to break this now. Note that SciPy 1.15 used install_rpath arguments that are implemented here, thus it would be fine, but it uses the $ORIGIN anchor on macOS too, and that does not work. See above. The behavior changes only with Meson 1.9.0 or later (older Meson versions did not export the required metadata), thus projects that pin the Meson version are fine.

@rgommers I think I added test cases for all scenarios we discussed. It would be great if you could test with packages that may be affected and that do not pin the meson-python version to any released version.