Load starlark files from soong

There are a number of instances where we are exporting information
from soong to bazel via soong_injection. This could be more bazel-centric
if the information was instead held in bzl files, and both bazel and
soong read it from there.

Add a starlark package that will run
//build/bazel/constants_exported_to_soong.bzl at initialization time,
and then results can be retreived with GetStarlarkValue.

Since changes to the starlark files mean that soong has to rerun,
add them as ninja deps.

Unfortunately, the starlark code has to be run at runtime rather than
pregenerating their results, because tests run from intellij wouldn't
go through any pregeneration steps. This means that starlark is run
multiple times during the build, once per test package and once per
primary builder invocation. (currently 3, could be reduced to 2 if we
made the symlink forest generation into its own standalone tool) The
starlark code we have so far in this cl is very fast, roughly half a
millisecond, so it's not a big deal for now, but something to keep an
eye on as we add more starlark constants.

Bug: 279095899
Test: go test
Change-Id: I1e7ca1df1d8d67333cbfc46e8396e229820e4476