Un-nest private implentations of IFileInfo and IDirectoryContents

[mgrosperrin]

As said in #142 (comment), the private implementations of IFileInfo and IDirectoryContents should be un-nest and made public.
I can submit a PR to do it.

[pranavkm]

👍 We got some of the PhysicalFileProvider ones made public in a recent PR. Would be nice to make the remainder public too.

[muratg]

@mgrosperrin Feel free to submit a PR! Thanks

[pranavkm]

For this change, would it make sense to move the non-primary API to a nested namespace?
For instance:

• Microsoft.AspNet.FileProviders.PhysicalFileProvider
• Microsoft.AspNet.FileProviders.Physical.PhysicalDirectoryInfo (this would match the project name).

This way we don't end up polluting the top level namespace when making the less common types public.

[muratg]

Yeah, I think it makes sense. Of course it's hard to find good names for the namespace.

[pranavkm]

I'll send a PR - we can discuss it as part of it.