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