Fetch files from a directory

Summary:
Mirror the File class into a Directory class,
with similar methods.

Allow to glob files in a directory and gets the result
as a collection of File instances.

Use cases

Nasqueron Notifications, Auth Grove and Zed can benefit from
autodiscovery mechanism where all files in a directory are
loaded as events, services or, for Zed, health checks.

Sites also use glob to find files like images in a directory.

With those new Directory and glob components, result will use
the File class, offering an unified interface to query file
and path information.

Test Plan:

• Unit tests provided
• Zed uses Directory::glob method for SiteHealth engine (see T1678).

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1678

Differential Revision: https://devcentral.nasqueron.org/D2500