Fix mod path when both mod file and mod dir exist

[Frederick888]

When both src/parent.rs and src/parent/ exist, construct_mod_path can be called with construct_mod_path('src/parent.rs', 'child'). In this case it should return src/parent/child.rs instead of nil.

Closes #60

[Frederick888]

Please add a test

Done 👌

[rouge8]

Thanks!