Bp2build: handle embedded structs as blueprint

For structs that are embedded, Blueprint does not nest under the
embedded name, flattening them into the original struct for blueprint
files (e.g.
https://cs.android.com/android/_/android/platform/build/blueprint/+/9fd2ed93dfb82baf7688c1b2b29aa4b2cc125721:proptools/unpack_test.go;l=402-431;drc=3adb2409648d6f8b25354ac47f083dae87731f10).
We should do the same for bp2build.

This will also allow us to embed structs for bp2build conversion
allowing more reuse.

Test: go test bp2build tests
Change-Id: I9ce088462adaf59bffa80bea76cd488e31f98e9d
diff --git a/bp2build/bzl_conversion.go b/bp2build/bzl_conversion.go
index f2f6b01..992cc1c 100644
--- a/bp2build/bzl_conversion.go
+++ b/bp2build/bzl_conversion.go
@@ -160,8 +160,15 @@
 		if shouldSkipStructField(field) {
 			continue
 		}
-
-		properties = append(properties, extractPropertyDescriptions(field.Name, field.Type)...)
+		subProps := extractPropertyDescriptions(field.Name, field.Type)
+		// if the struct is embedded (anonymous), flatten the properties into the containing struct
+		if field.Anonymous {
+			for _, prop := range subProps {
+				properties = append(properties, prop.properties...)
+			}
+		} else {
+			properties = append(properties, subProps...)
+		}
 	}
 	return properties
 }