Bp2build support for multiple product configs

Create a
build/bazel/product_config/generated/products/<product_name>/BUILD
file that contains the platform definitions needed for
a particular product. Currently we just create it for the
current lunch target, but the idea is that eventually when
all product config is in starlark, all the products will
have their platform definitions in the tree at once.

Bug: 249685973
Test: Presubmits
Change-Id: I08c82ff28dcf62f09d3b1d2e3186a6b961e12f6e