Integrate Starlark-based configuration runner

If RBC_PRODUCT_CONFIG variable is set, obtain product configuration
variables by converting product configuration makefiles to Starlark
files and then executing them.
Also, introduce RBC_NO_PRODUCT_GRAPH variable to suppress product graph
generation. We cannot generate product graph with Starlark, so this
option allows to verify that the rest of the contents of the generated
Ninja files remains the same when Starlark-based converter is used.
This allows to perform the regression testing, i.e. running
`RBC_NO_PRODUCT_GRAPH=t DISABLE_ARTIFACT_PATH_REQUIREMENTS=t m nothing`
and
`RBC_PRODUCT_CONFIG=t m nothing`
should generate identical *.ninja files.

Bug: 181797530
Test: Manual
Change-Id: Ic6173a9640f32766b71c02a2b1833ce7a278e4cc