Adding `host_exports` (like `run_exports` except between `build` & `host` only)

[jakirkham]

run_exports has been immensely helpful. Just to summarize it run_exports:

• Allows packages with knowledge of what they need to express it for easy downstream consumption
  • Packages that are part of the build can add dependency to run
  • weak only happens when the dependency is added to host (not build)
  • strong happens when a dependency is added to build or host
  • Dependencies added can express ABI compatibility

However there are still some cases that are not covered. In particular there are cases where a dependency added to build needs to add itself to host, but there is no need to add it to run (in fact this would often be better to avoid). IOW run_exports/strong does not meet this need. Some examples include:

• Header-only dependencies of a package
  • https://github.com/conda/conda-build/issues/3796#issuecomment-549431589
• Tighter alignment in build & host of compiler dependencies (like OpenMP)
  • https://github.com/conda-forge/openmp-feedstock/issues/126
• Compiler runtime libraries that are statically linked (like cudart)
  • https://github.com/conda-forge/staged-recipes/pull/21723#issuecomment-1405444862

Solving these cases would involve a host_exports that allows adding other dependencies to host. Could imagine to use cases for a package providing host_exports

• host_exports/weak (default)
  • Add dependencies to host if providing package is added to host
  • Solves the header-only dependency case above for a library
• host_exports/strong (opt-in)
  • Add dependencies to host if providing package is in build or host
  • Handles the compiler dependency alignment problem (with OpenMP)
  • Handles statically linked compiler libraries (like cudart)
  • Of most use for compiler use cases

[jakirkham]

cc @xhochy (for awareness)