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/build_conversion.go b/bp2build/build_conversion.go
index b1a6e2c..4a0eeea 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -635,6 +635,7 @@
 			// Ignore zero-valued fields
 			continue
 		}
+
 		// if the struct is embedded (anonymous), flatten the properties into the containing struct
 		if field.Anonymous {
 			if field.Type.Kind() == reflect.Ptr {