drop google/go-cmp dependency

[alexandear]

The PR replaces the usage of github.com/google/go-cmp/cmp package with reflect.

[mvdan]

Sorry but this is incorrect. We want to compare the types described by the syntax tree, so e.g. Field1 []int vs Field2 []string. We need to do a deep comparison of the syntax trees; a basic comparison via reflect.TypeOf will incorrectly think the two types there are equal, as they are both *go/ast.ArrayType.

I can see how the comment in the code is slightly misleading, and we don't have tests which catch the edge case I described above, so I will send a patch for the sake of those. But the proposed change to the code isn't valid as far as I can see.

[ccoVeille]

Does it mean some unit tests are missing then?

[mvdan]

Yes, some "fields should not be merged due to slightly different types" tests are missing as I described above. I will send a patch for those.