Source distributions contain rts.cabal

Summary

GHC source distributions contain the file rts.cabal, which is generated by the build. That file should probably be excluded from source distributions.

Steps to reproduce

$ tar -tf ghc-8.8.1-src.tar.xz  | grep rts.cabal
ghc-8.8.1/rts/rts.cabal.in
ghc-8.8.1/rts/rts.cabal

I also confirmed that this is still happening in HEAD with Hadrian:

$ tar -tf _build/source-dist/ghc-8.9.0.20190926-src.tar.xz | grep rts.cabal
_build/source-dist/ghc-8.9.0.20190926-src/rts/rts.cabal
_build/source-dist/ghc-8.9.0.20190926-src/rts/rts.cabal.in

Expected behavior

The tarballs should not include rts.cabal.

Environment

• GHC version used: 8.6.4 and 8.8.1