Introduce module type aconfig_declarations_group

In order to easily manage aconfig_declarations modules and their
corresponding codegen modules, this change introduces a new module type
`aconfig_declarations_group`. The module enables listing codegen modules
by language, and correctly depend on the desired output files using
appropriate tags.

e.g. for an aconfig_declarations_group module "some_group", the rdeps of
the module can:
- gather all intermediates cache files of the aconfig_declarations with
  ":some_group" tag.
- gather generated srcjar files of the listed java_aconfig_library
  modules with ":some_group{.srcjars}" tag.

Output tag support for cc modules and rust modules will be added in
future changes.

Test: m nothing --no-skip-soong-tests
Bug: 320492079
Change-Id: I93d737577f8d00198ed91048dd6e81ef238193cb

aconfig/codegen/Android.bp

@@ -17,6 +17,7 @@ bootstrap_go_package {
         "soong-rust",
     ],
     srcs: [
+        "aconfig_declarations_group.go",
         "cc_aconfig_library.go",
         "init.go",
         "java_aconfig_library.go",
@@ -24,6 +25,7 @@ bootstrap_go_package {
         "testing.go",
     ],
     testSrcs: [
+        "aconfig_declarations_group_test.go",
         "java_aconfig_library_test.go",
         "cc_aconfig_library_test.go",
         "rust_aconfig_library_test.go",