Add ability to specify library directories for target rpaths

[vyasr]

Description

Python extension modules built using scikit-build and rapids-cmake are typically designed around a model where a C++ library is built and the resulting Cython extension modules must link to it. While it is in principle possible to use both scikit-build and rapids-cmake with only Cython, that is not the case for any of the major RAPIDS packages. When the C++ component is built via scikit-build and installed into the final package, it must also be discoverable by the extension modules. Currently this is accomplished by propagating a variable RAPIDS_CYTHON_CREATED_MODULES from rapids_cython_create_modules that calling functions can use to get the list of targets whose RPATHs need updating, and setting these RPATHs is the responsibility of the caller. However, this approach is error-prone and frequently results in incorrect RPATH specifications.

This PR automates the process by having the calling code provide a set of directories where compiled libraries may be placed. The RPATHs of all extension modules built via rapids_cython_create_modules are then automatically updated to include those library directories.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

Here are some minor changes.

I think this is a good starting place but we should change the separation of concerns.

I think we should have create_modules.cmake just add the ORIGIN rpath entry, and have a secondary function that you call providing a target, and a set of directories and it will add those as INSTALL_RPATHS.

So instead of having the logic look like:

set_lib_dirs( ..... ) # propagate stuff via side channels to create_modules
create_modules( )

We have

create_modules()
set_rpath_entries(<cython_target> <dirs>)

[robertmaynard]

Overall looks great, have a couple of minor suggestions.

[vyasr]

@gpucibot merge