[cmake] refactor: cpmfile, deps prefetch, force system and more (#322)

CPM Dependencies are now managed in a singular json file, where each can be properly prefetched at-will via `tools/cpm-fetch.sh <packages...>`, or all at once via `tools/cpm-fetch-all.sh`.

Adds docs for CPMUtil as well.

Also adds `<package>_FORCE_{BUNDLED,SYSTEM}` overrides

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/322
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

externals/nx_tzdb/cpmfile.json

@@ -0,0 +1,8 @@
+{
+    "tzdb": {
+        "package": "nx_tzdb",
+        "url": "https://github.com/crueter/tzdb_to_nx/releases/download/250725/250725.zip",
+        "hash": "8f60b4b29f285e39c0443f3d5572a73780f3dbfcfd5b35004451fadad77f3a215b2e2aa8d0fffe7e348e2a7b0660882b35228b6178dda8804a14ce44509fd2ca",
+        "version": "250725"
+    }
+}