Allow custom vendors

[JonathanLorimer]

Closes https://github.com/tweag/gomod2nix/issues/20

I came across a go package that uses replace directives to reference local files. Unfortunately this doesn't generate an entry in the go.sum file. Therefore I need the ability to splice in the required directories into /vendors. This parameter would facilitate that.

[gregistech]

Trying this PR have resulted with these issues:

error: attribute 'overlays' missing

       at /nix/store/g6as3z4mjh80ckzg9r3r9zgmgjwnmg48-source/flake.nix:32:20:

           31|     let
           32|       overlays = [ gomod2nix.overlays.default ];
             |                    ^
           33|     in flake-utils.lib.eachDefaultSystem (system:

Switched back to old gomod2nix.overlay and then:

error: attribute 'fetch' missing

       at /nix/store/vkdvav33cfvgr10xj5rhym0i98ba8ybw-source/builder/default.nix:54:28:

           53|                 src = fetchgit {
           54|                   inherit (meta.fetch) url sha256 rev;
             |                            ^
           55|                   fetchSubmodules = true;