ghc-pkg should not accept unversioned depends
Historically ghc-pkg accepted a format where the "depends:" field did not require version numbers. Then ghc would just resolve it to some installed version. However now that ghc bakes in the version number to libraries when it compiles them it really crucial that a package gets linked with exactly the versions of packages it was built against. Allowing ghc-pkg to accept unversioned depends and resolving them to some random installed version is just asking for trouble.
It's not too serious since Cabal always uses versioned depends, however I just noticed that older autoconf/automake build systems do not.
So, in summary allowing undersioned depends is just asking for trouble and ghc-pkg should reject them.