Using BUILD_OVERLAY=true fails in restructured branch

[petefoth]

Using BUILD_OVERLAY=true fails the second device with a load of already defined errors: stuff in tmp/merged also exists in normal $SRCDIR

PRODUCT_SOONG_NAMESPACES=device/sony/tama-common hardware/google/interfaces hardware/google/pixel hardware/lineage/interfaces/power-libperfmgr hardware/qcom-caf/common/libqti-perfd-client hardware/qcom-caf/sdm845 vendor/qcom/opensource/commonsys/display vendor/qcom/opensource/commonsys-intf/display vendor/qcom/opensource/display vendor/qcom/opensource/data-ipa-cfg-mgr-legacy-um vendor/qcom/opensource/dataservices hardware/qcom-caf/wlan
============================================
error: merged/external/go-cmp/Android.gen.bp:4:1: module "go-cmp" already defined
       external/go-cmp/Android.gen.bp:4:1 <-- previous definition here

Originally posted by @petefoth in #678 (comment)