Skip import-dirs for empty libraries

[aherrmann]

The issue was discovered on Windows where a Bazel Haskell build with Bazel 7 and rules_haskell 1.0 failed to build the package @stackage//:retry which depends on the empty package @stackage//:mtl-compat due to Cabal complaining about a missing _iface directory at the location configured by the import-dirs entry in the package-db. This change resolves that issue.

• Skip import-dirs for empty libraries
• fix indentation