add setup/matching-platform

Put a function that was implemented in two places in one place.
raco pkg install: make conflict checking sensitive to platform specifier
raco setup: add `--check-pkg-deps` for checking specified collections

This flag enables scalable package dependency checking to a large
degree, although dependencies related to documentation cross
references can escape detection.
raco setup: add `--pkgs` argument

The arguments to `--pkgs` specify collections to set up as the collections
that are (partially) provided by a set of packages.

Closes PR 14396
Define SCHEME_PLATFORM_LIBRARY_SUBPATH for some architectures on OpenBSD