Rearrange public API headers

[msimberg]

As discussed with @hkaiser and @aurianer. We decided to change hpx/x.hpp headers to include both distributed and local headers wherever it fits (like future.hpp and runtime.hpp, more in the future), and the distributed header is conditionally included depending on if the distributed runtime is enabled in the configuration.

Users that really always only want the local functionality should include hpx/local/x.hpp. Not everything has a header in hpx/local or hpx/distributed as it doesn't make sense for everything. Should we add dummy headers hpx/local/x.hpp/hpx/distributed/x.hpp (that would just include the same as hpx/x.hpp) for some sort of symmetry? (I vote no, since it's just more to maintain and the cases where the split is needed will be listed in the documentation.)

Extra: I moved most of the API headers to the include module where it's a bit easier to have an overview of them.

[msimberg]

Flyby: I had completely forgotten to add most of the synchronization primitives to the API page, now they're there.

Question: Should we already add the trio channel, barrier, latch to the hpx::distributed namespace (and headers)? I see no reason why not. These are most likely some of the more stable distributed LCOs we have. I can't judge how much else from the collectives modules should be added to a non-experimental namespace though.

Edit: I've now added headers for barrier, channel, and latch.

[biddisco]

I find this a bit strange. Why not have

#include <hpx/x.hpp>

and inside the x.hpp add

#if defined(HPX_HAVE_DISTRIBUTED_RUNTIME)
#include <hpx/distributed/x.hpp>

and not leave it to chance that the user 'knows' every module that has local and distributed versions?

[msimberg]

I find this a bit strange. Why not have

#include <hpx/x.hpp>

and inside the x.hpp add

#if defined(HPX_HAVE_DISTRIBUTED_RUNTIME)
#include <hpx/distributed/x.hpp>

and not leave it to chance that the user 'knows' every module that has local and distributed versions?

I'm quite honestly unsure if we agree or disagree here. That sounds exactly like what I've done. See e.g. hpx/future.hpp: https://github.com/STEllAR-GROUP/hpx/blob/eed6733f835bfbc19242e419d0aed5832b43c824/libs/include/include/hpx/future.hpp. Are you saying that every hpx/x.hpp should also have a hpx/local/x.hpp and hpx/distributed/x.hpp no matter if there actually is a distinction? The users who don't want to care can include just hpx/x.hpp, and the users who absolutely want to avoid including distributed features would have to (currently) know which headers have a local variant (or they just check in the documentation).

[biddisco]

Sorry. I completely misunderstood the PR and got the diffs the wrong way around. Please proceed and ignore my comments.

[hkaiser]

LGTM, thanks!