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

java/BUILD.bazel

diff --git a/java/BUILD.bazel b/java/BUILD.bazel
index 1b45507..14838ae 100644
--- a/java/BUILD.bazel
+++ b/java/BUILD.bazel
@@ -4,11 +4,6 @@
 sh_test(
     name = "linkage_monitor",
     size = "large",
-    tags = [
-        # Exclude this target from wildcard expansion (//...) because it may
-        # take unnecessary time.
-        "manual",
-    ],
     srcs = [
         ":linkage_monitor.sh",
     ],
@@ -28,6 +23,11 @@
         "*/pom.xml",
         "test/linkage-monitor-check-bom/pom.xml",
     ]),
+    tags = [
+        # Exclude this target from wildcard expansion (//...) because it may
+        # take unnecessary time.
+        "manual",
+    ],
 )
 
 test_suite(