inputMap: add missing fallback to repoData.rev

[jbgi]

to properly support inputMap keys without rev.

Before this commit I got errors like:

error: attribute 'ref' missing

       at /nix/store/byfll3yha4clnqif577vv3mlii5f9p5h-source/lib/call-cabal-project-to-nix.nix:227:71:

          226|               then
          227|                 (if inputMap.${repoData.url}.rev != (__trace repoData repoData.ref)
             |                                                                       ^
          228|                   then throw "${inputMap.${repoData.url}.rev} may not match ${repoData.ref} for ${repoData.url} use \"${repoData.url}/${repoData.ref}\" as the inputMap key if ${repoData.ref} is a branch or tag that points to ${inputMap.${repoData.url}.rev}."
       Did you mean rev?

when specifying inputsMap keys from only the url (no rev).

[fumieval]

@michaelpj @hamishmack Could you take a look? This is blocking us from upgrading haskell.nix for a cabal project