allow out_data_dirs to be referenced through output groups (#1451)

This change allows one to easily reference the content of folders listed
in `out_data_dirs` through an output group, instead of relying on
`gen_dir` & `genrule`

Author: chouquette

foreign_cc/private/framework.bzl

@@ -592,7 +592,8 @@ def cc_external_rule_impl(ctx, attrs):
         outputs.out_binary_files +
         outputs.libraries.static_libraries +
         outputs.libraries.shared_libraries +
-        [outputs.out_include_dir] if outputs.out_include_dir else []
+        ([outputs.out_include_dir] if outputs.out_include_dir else []) +
+        (outputs.data_dirs if outputs.data_dirs else [])
     )
     output_groups = _declare_output_groups(installdir_copy.file, output_groups)
     wrapped_files = [
@@ -842,6 +843,7 @@ _Outputs = provider(
         out_include_dir = "Directory with header files (relative to install directory)",
         out_binary_files = "Binary files, which will be created by the action",
         libraries = "Library files, which will be created by the action",
+        data_dirs = "Directory containing additional files generated by the build",
         declared_outputs = "All output files and directories of the action",
     ),
 )
@@ -898,6 +900,7 @@ def _define_outputs(ctx, attrs, lib_name):
         out_include_dir = out_include_dir,
         out_binary_files = out_binary_files,
         libraries = libraries,
+        data_dirs = out_data_dirs,
         declared_outputs = declared_outputs,
     )