Cabal cannot upload hs-source-dirs that only have autogen-modules

**Describe the bug**

If a Cabal package has a source directory listed in `hs-source-dirs` that only contains `autogen-modules` then the package cannot be uploaded without a workaround. Specifically: `cabal sdist` will omit the entire source directory when creating the sdist and then `cabal upload` and/or Hackage rejects the package because the sdist is missing the source directory listed in `hs-source-dirs`.

**To Reproduce**

Steps to reproduce the behavior:

```bash
$ cat > autogen-issue.cabal <=1.10

library
build-depends: base >=4.13 && <4.14
hs-source-dirs: src
other-modules: Example
autogen-modules: Example
default-language: Haskell2010
EOF

$ mkdir src

$ cat > ./src/Example.hs <