Reorganize upb file structure

This change moves almost everything in the `upb/` directory up one level, so
that for example `upb/upb/generated_code_support.h` becomes just
`upb/generated_code_support.h`. The only exceptions I made to this were that I
left `upb/cmake` and `upb/BUILD` where they are, mostly because that avoids
conflict with other files and the current locations seem reasonable for now.

The `python/` directory is a little bit of a challenge because we had to merge
the existing directory there with `upb/python/`. I made `upb/python/BUILD` into
the BUILD file for the merged directory, and it effectively loads the contents
of the other BUILD file via `python/build_targets.bzl`, but I plan to clean
this up soon.

PiperOrigin-RevId: 568651768

objectivec/BUILD.bazel

diff --git a/objectivec/BUILD.bazel b/objectivec/BUILD.bazel
index 70727cc..cb0d7c8 100644
--- a/objectivec/BUILD.bazel
+++ b/objectivec/BUILD.bazel
@@ -42,8 +42,8 @@
         for wkt in _OBJC_WKT_NAMES
         for ext in _OBJC_EXTS
     ]),
-    tools = ["//:protoc"],
     tags = ["manual"],
+    tools = ["//:protoc"],
 )
 
 staleness_test(