Prevent duplicate entries in ctx.include_tops

The include_tops hints are global for the whole parse
context (which is probably also something to fix), which
means that if an include_top hint is duplicated there will
be duplicated keys generated in the _entry starlark dictionary.

Bug: 193566316
Test: go test
Change-Id: I01a0546ac9be91ef46c5248e87e1a40e0f211193