Implement bp2build for the `package` module

* We are interested mostly in the conversion the `default_applicable_licenses`
  attribute, as `default_visibility` cannot be handled until every module's
  `visibility` is handled.
* Several referenced license modules had to be manually enabled for
  conversion, and likewise a few trivial Android.bp containing only
  package and license modules.
* As Bazel allows only a single `package` rule, the package rules in a
  manually converted BUILD.bazel files were removed (in
  external/protobuf and prebuilts/clang/host/linux-x86 trees).
* The converted package rule is emitted before the `load` statements per
  Bazel documentation.

Bug: 190817312
Test: treehugger
Change-Id: If8bf6fee1580177de3bb402923615bcd48923ed2
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index a3f618e..7c9af1a 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -67,6 +67,7 @@
         "license_kind_conversion_test.go",
         "linker_config_conversion_test.go",
         "ndk_headers_conversion_test.go",
+        "package_conversion_test.go",
         "performance_test.go",
         "prebuilt_etc_conversion_test.go",
         "python_binary_conversion_test.go",