Ext4Pkg: Add a directory entry tree.

This helps us track directories and directory entries,
which helps us getting a general idea of how the filesystem
looks. In the future, it might serve as a directory cache.
Right now, it only lets us know which name "." and ".." may refer
to, which fixes a EFI_FILE_PROTOCOL::GetInfo() bug that got triggered
by ShellPkg, in some MdePkg code.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>