Use selective imports when importing platform-specific modules (core.sys)

[Reavershark]

Why

This makes it easier to add support for new platforms to druntime, and maintain platform-specific differences in existing ones. It better reveals the actual dependencies of each module by avoiding implicit public imports.

Progress

This is done for linux and most posix platforms.
Most imports for darwin, solaris and windows are still TODO.

Future work

I'd also like to attempt this with core.stdc. That would (in combination with this pr) open up the possibility for developers working with a Freestanding platform to get parts of druntime to work without needing an entire supported libc, just specific functions.
Also, contrary to what druntime assumes, not every platform has concepts like a filesystem, stdio, processes... This could help in determining what modules have those features as hard dependencies, and what modules could also work without them on such platforms.

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#20632"

[dkorpel]

Is this ready for review? (Note that you don't have to do all platforms in one go, incremental work is usually preferred over huge Pull Requests)

[Reavershark]

Is this ready for review?

I think it is. I rebased and will mark this as such once ci finishes.

(Note that you don't have to do all platforms in one go, incremental work is usually preferred over huge Pull Requests)

I'll keep that in mind next time.
I'm not doing windows as it's out of my comfort zone.

[dkorpel]

This is not in scope of this Pull Request, but this code is iffy. It assumes that 'not Windows' = posix and imports posix.dlfcn while it used to selectively import RTLD_NOLOAD from core.sys.{dragonflybsd, linux, netbsd ...}.dlfcn. It should probably do else version (posix) { dlopen ... } else static assert(0, "unsupported platform").

[Reavershark]

Ah right, RTLD_NOLOAD isn't posix, but every platform has it, and rather than being defined in their own core.sys modules they are all defined in core.sys.posix with versions and publicly imported into the respective core.sys.platorm from there. That had me confused. No reason to remove it.
And yes that's one weird version block.