Cleanup: Split compatibility-date.capnp into its own capnp_library.

This allows us to clean up dependencies of some internal libs. It's especially nice to eliminate the dependency on the generated file `trimmed-supported-compatibility-date.txt`.

src/workerd/io/BUILD.bazel

@@ -136,19 +136,23 @@ capnp_embed(
     src = ":trimmed-supported-compatibility-date-gen",
 )
 
+# TODO(cleanup): Split this target up further, as many depnedents don't need all the files here.
+#   Avoiding the dependency on supported-compatibility-date.capnp is often helpful due to the
+#   transitive dependency on `trimmed-supported-compatibility-date.txt` which is dynamically
+#   generated.
 wd_cc_capnp_library(
     name = "capnp",
     srcs = [
         "actor-storage.capnp",
         "cdp.capnp",
-        "compatibility-date.capnp",
         "supported-compatibility-date.capnp",
         "worker-interface.capnp",
     ],
     visibility = ["//visibility:public"],
     deps = [
         ":outcome_capnp",
         ":script_version_capnp",
+        ":compatibility_date_capnp",
         ":trimmed-supported-compatibility-date",
         "@capnp-cpp//src/capnp/compat:http-over-capnp_capnp",
     ],
@@ -166,6 +170,12 @@ wd_cc_capnp_library(
     visibility = ["//visibility:public"],
 )
 
+wd_cc_capnp_library(
+    name = "compatibility_date_capnp",
+    srcs = ["compatibility-date.capnp"],
+    visibility = ["//visibility:public"],
+)
+
 [kj_test(
     src = f,
     deps = [