cabal haddock can fail if the build artifacts were deleted

Suppose you run `cabal build` on a project, and then later on the build folder gets deleted and you run `cabal haddock`. This will cause a build failure because the `reusingGHCCompilationArtifacts` function expects the artifacts to be there. If instead we simply don't try to copy artifacts that don't exist, haddocking succeeds.

I am fixing in #10992.