Add a script for generating the 'extra-source-files' field.

Cabal/misc/gen-extra-source-files.sh

+#! /bin/sh
+
+find tests -type f \( -name '*.hs' -or -name '*.c' -or -name '*.sh' \
+    -or -name '*.cabal' -or -name '*.hsc' \) -and -not -regex ".*/dist/.*" \
+    | awk '/Check.hs$|UnitTests|PackageTester|autogen|PackageTests.hs|CreatePipe/ { next } { print }'