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
diff --git a/build_defs/BUILD.bazel b/build_defs/BUILD.bazel
index 0a45eae..ea8fa18 100644
--- a/build_defs/BUILD.bazel
+++ b/build_defs/BUILD.bazel
@@ -89,7 +89,7 @@
     match_any = [
         ":config_win32",
         ":config_win64",
-    ]
+    ],
 )
 
 config_setting(
@@ -107,7 +107,7 @@
     match_any = [
         ":config_osx_aarch64",
         ":config_osx_x86_64",
-    ]
+    ],
 )
 
 # Internal testing: