Delay enumeration of directory until IDirectoryContents enumerated

[davidfowl]

Not sure why we do this inline, but I was profiling the static file middleware and saw enumeration even though it's just checking for directory existence:

https://github.com/aspnet/FileSystem/blob/dev/src/Microsoft.Extensions.FileProviders.Physical/PhysicalFileProvider.cs#L219-L236

https://github.com/aspnet/StaticFiles/blob/dev/src/Microsoft.AspNetCore.StaticFiles/DefaultFilesMiddleware.cs#L69-L95

[Yves57]

I just had a somewhat similar problem in a WebDAV middleware implementation: I just want to check a path validity, and if this path represents a file or a directory.
Calling GetFileInfo works for the files but not for the directories. So if it fails, I have to call GetDirectoryContents to check if it is a directory. And of course it enumerates all child entries for nothing in my case.

So just for curiosity, why GetFileInfo does not return a valid IFileInfo for a directory? For performance reasons in case of the path is invalid?

[natemcmaster]

Might be room for even more improvement that #237. See #237 (comment)