WIP: [cmake] allow fetching deps with CPM instead of submodules

[crueter]

Extremely WIP, thunks and tests are untested since I don't have an aarch64 development environment on me at the time. Will document this later.

Main TODOs:

• copy other modules over to CPM
• thunks wtf
• test
• robin map????????

Signed-off-by: crueter [email protected]

[Sonicadvance1]

I'm not sure how I feel about this one yet.

[crueter]

I'll document more thoroughly when I actually finish it, but the gist is

• submodules are annoying (I have already been bitten by forgetting to git submodule update multiple times), whereas with this each commit sha/tag is its own cache entry in .cache/cpm/<packagename>. This also has the benefit of every build being 100% reproducible
• this uses source archives instead of full Git trees, which is net faster as it doesn't need to include the .git directory for each

[lioncash]

tbh, it'd be nice if we could track the main repo for robin-map, especially since it seems that we're 2 minor versions behind now (i.e. 1.2.1, but 1.4.1 is out, which does seem to have some performance improvements, though small, and also unbreaks systems using CMake 4.0+)

[Sonicadvance1]

We have one patch on top of upstream so we can't track upstream. FEX-Emu/robin-map@d5683d9

[lioncash]

oh, bleh. I guess in that case it would be good to reapply on top of the latest release then (if possible), so we don't have an eventual build break

[crueter]

To be honest I wish we could just use something like unordered_dense instead

In CPM land, applying patches is as simple as creating a patch and applying it with the PATCHES option. That's another benefit of using it over submodules.

[crueter]

This needs a lot more time to bake, so I'm closing it for now and keeping it as a future reference. Self-notes:

• A separate PR should be made for preferring system dependencies if available. This makes the CPM porting work easier and reduces this PR's size--this depends on [externals] use ankerl::unordered_dense over tsl #5162 to avoid the robin-map patching thing.
• MERGE CONFLICTS
• Will probably want each CPM dependency to have an option like <name>_DIR where users can prefetch the dependency and use it in the same way you can submodules (see https://gpo.zugaina.org/app-emulation/FEX)
• Patches... ideally, none are used, but you can't win it all I guess. Could go with the same CPMUtil route of .patch/<packagename>/0000-patchname.patch perhaps.