RFC re-parent deduped omnibus roots back to origin cell
RFC re-parent deduped omnibus roots back to origin cell

Summary:
Context: S218964
It looks like D24879352 is causing issues for cross-cell native roots: [full log](https://www.internalfb.com/intern/sandcastle/log/?instance_id=9007199616627734&step_id=9007202708873740&step_index=5&name=Build#275-288)

`filesystem cell '' must match build target cell: fbsource//third-party/pypi/cffi/1.14.4:_cffi_backend#omnibus-root-79d0d73dc,platform007-clang,py3.7-platform007,v8fcb3a7`

It seems like this is because the old version would return a build target based on the binary name, and this now returns a target based on the library name. But for a target based on a library name, the code on [`Omnibus.java:1010`](https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/build_infra/buck_client/src/com/facebook/buck/cxx/Omnibus.java?commit=ffe726617b0fa836ff85ef6d674270a6134cb6b7&lines=1010-1023) that tries to construct the rule with the wrong `projectFilesystem` (belonging to the binary cell instead of the library cell, and also the configs aren't going to be quite right either since the cxxConfig platform etc are all going to be from `fbcode` instead of `xplat`)

I think in most cases where we go cross-cell we have to all the way back through the target resolution -> Description -> createBuildRule process, but I imagine that would slow this part down quite a bit. So instead, in this diff I'm rooting the generated de-duped target name on the dummy omnibus and preserving the library name (root) in the flavor, similar to the non-deduplicated version (which was rooted on the binary name). This appears to work locally, but open to feedback or other ways to fix the issue.
Alias cffi, pycparser
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Reviewed By: andrewjcg

fbshipit-source-id: 3bfbd72ee25a5d739a9e5cf305d21ae023d61d1d

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