Prior to 0.7.1, a module_path of `./lib` would result in a package
containing files like this (using `test/app8` as an example):

    lib/app8.node

The switch to tar 3 in 0.7.1 causes the same package to end up like
this:

    package.json
    app8.cc
    binding.gyp
    index.js
    README.md
    lib/app8.node

which causes a problem during remote binary installation:

    node-pre-gyp ERR! Pre-built binaries not installable for PACKAGE@0.0.1 and node@8.3.0 (node-v57 ABI, glibc) (falling back to source compile with node-gyp)
    node-pre-gyp ERR! Hit error ENOTDIR: Cannot cd into '..dirs/package/lib'

This commit fixes this problem and allows for single-level module_path
values.

But don't compare Linux to Apple.

This should work everywhere except AIX.

This is based on what showed up during an Appveyor run. Someone with
more Win32 module building experience should verify that these are
expected and correct.