bp2build: Add support for export_.*headers props

Soong supports export_.*_headers properties, the libraries contained in
this list must also be within a shared/static/whole_static/header libs
property. For bp2build, we eliminate this duplication. The libraries
not listed in an export_.*_headers property will migrate to an attribute
prepended with implementation_, those in export_.*_headers will not have
a prefix.

Test: build/bazel/ci/bp2build.sh
Test: build/bazel/ci/mixed_libc.sh
Bug: 198241472
Change-Id: I3eb84c983ec5d241c8a568e411dfd5619d3184a7
diff --git a/bp2build/conversion.go b/bp2build/conversion.go
index 354abf6..0a86a79 100644
--- a/bp2build/conversion.go
+++ b/bp2build/conversion.go
@@ -142,7 +142,7 @@
 }
 
 func shouldSkipStructField(field reflect.StructField) bool {
-	if field.PkgPath != "" {
+	if field.PkgPath != "" && !field.Anonymous {
 		// Skip unexported fields. Some properties are
 		// internal to Soong only, and these fields do not have PkgPath.
 		return true