always generate posix paths in manifests

[glguy]

I've adapted the manifest builder to always build paths with forward slash. My reading of the documentation is that Python on Windows will translate these automatically.

@kityansiu does this fix generating the turnstile example for you?

[weisenje]

@glguy I get this error (running on Windows) - let me know if I'm doing something wrong.
@kityansiu

$ rack manifest build manifest.yaml turnstile
Traceback (most recent call last):
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\Scripts\rack", line 69, in <module>
    func(args)
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\lib\site-packages\rack\__init__.py", line 877, in dispatch_manifest_build
    build_manifest_driver(Path(args.config), Path(args.zipfile))
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\lib\site-packages\rack\__init__.py", line 506, in build_manifest_driver
    builder.manifest(manifest_path, Path(outdir).joinpath(f'manifest.yaml'))
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\lib\site-packages\rack\__init__.py", line 470, in manifest
    self.manifest(base_path.joinpath(path), topath)
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\lib\site-packages\rack\__init__.py", line 470, in manifest
    self.manifest(base_path.joinpath(path), topath)
  File "C:\Users\200005868\projects\rack\RACK\cli\venv2\lib\site-packages\rack\__init__.py", line 479, in manifest
    step['model'] = str(PosixPath(dirname).joinpath(PosixPath(path).name))
  File "C:\Users\200005868\Miniconda3\lib\pathlib.py", line 1074, in __new__
    raise NotImplementedError("cannot instantiate %r on your system"
NotImplementedError: cannot instantiate 'PosixPath' on your system

[glguy]

Well shoot. I was hoping that by using the posix-specific path class it wouldn't auto detect and use windows path separators. Back to the drawing board <_<

[glguy]

@weisenje after reading more documentation (and testing on a spare laptop) I think this commit is the fix