Lookup main-is C sources in hs-source-dirs

In the 2decb0e7 refactor we stopped
looking for non-Haskell `main-is` files in the hs-source-dirs and other
appropriate directories. This commit fixes that oversight.

Even if it is not intuitive that main-is-C-sources are searched in the
hs-source-dirs, we don't wish to break users relying on this behaviour
as there does not exist that strong of a motivation to do so.

Fixes #10168

Co-authored-by: sheaf <sam.derbyshire@gmail.com>