Wrong path to flow generated files on windows

**Intended outcome:**
On windows absolute path with disk name like `d:/some_project` is being treated as a relative path resulting in `d:/some_project/d:/some_project` path. If I replace`path.posix.relative` with `path.relative`, everything works as usual

**Actual outcome:**
Here the paste of what happens with path.posix.relative on windows

https://pastebin.com/49tFHfJm

**How to reproduce the issue:**
I don't have windows machine sadly.

**Versions**
most current version. Offending line is https://github.com/apollographql/apollo-tooling/blob/6a24e644f68039aa20b20397e0d7962f61f8b4a0/packages/apollo/src/generate.ts#L109

Start at packages/apollo/src/generate.ts, line 109, and inspect how path.posix.relative handles a Windows absolute path such as d:/some_project. Verify the generated-file path no longer duplicates the absolute path, using the pastebin example or an equivalent Windows environment check.

Written by the indexing model from the issue text.