Move cpp_utils to compilation_contexts (#109)

Move cpp_utils from the cc_common.compile private_hdrs parameter
to the compilation_contexts parameter in order to fix the
cpp_utils header include paths when Emboss is an external repository.

Fixes #108

build_defs.bzl

@@ -181,8 +181,7 @@ def _cc_emboss_aspect_impl(target, ctx):
         transitive = [
                          dep[EmbossCcHeaderInfo].transitive_headers
                          for dep in ctx.rule.attr.deps
-                     ] +
-                     [runtime_cc_info.compilation_context.headers],
+                     ],
     )
     (cc_compilation_context, cc_compilation_outputs) = cc_common.compile(
         name = ctx.label.name,
@@ -191,6 +190,7 @@ def _cc_emboss_aspect_impl(target, ctx):
         cc_toolchain = cc_toolchain,
         public_hdrs = headers,
         private_hdrs = transitive_headers.to_list(),
+        compilation_contexts = [runtime_cc_info.compilation_context],
     )
     return [
         CcInfo(compilation_context = cc_compilation_context),