product_variables srcs in prebuilt_etc

bp2build conversion handles product_variable srcs

bug: 228353067
Test: prebuilt_etc_conversion_test.go
Change-Id: I82d3a384ee14d4e981d502dd9eb824c87d5ae2c7
diff --git a/bazel/configurability.go b/bazel/configurability.go
index e1cdd4a..7ff202b 100644
--- a/bazel/configurability.go
+++ b/bazel/configurability.go
@@ -295,10 +295,11 @@
 )
 
 // ProductVariableConfigurationAxis returns an axis for the given product variable
-func ProductVariableConfigurationAxis(variable string) ConfigurationAxis {
+func ProductVariableConfigurationAxis(variable string, outerAxis ConfigurationAxis) ConfigurationAxis {
 	return ConfigurationAxis{
 		configurationType: productVariables,
 		subType:           variable,
+		outerAxisType:     outerAxis.configurationType,
 	}
 }
 
@@ -309,6 +310,8 @@
 	// some configuration types (e.g. productVariables) have multiple independent axes, subType helps
 	// distinguish between them without needing to list all 17 product variables.
 	subType string
+	// used to keep track of which product variables are arch variant
+	outerAxisType configurationType
 }
 
 func (ca *ConfigurationAxis) less(other ConfigurationAxis) bool {