Reviewed by Eric Seidel.

        Clean up of the filesystem-related modules used in webkitpy.
        I've added relpath() to the filesystem interface, modified
        ospath.relpath() so that it could work with the filesystem
        interface, and modified the fileset* routines to use the
        filesystem interface consistently.

        This patch also adds a close() routine to the fileset routines
        to indicate that the caller is done with the fileset. This
        allows zipfileset to clean up after itself when it creates
        tempfiles to store downloads.

        https://bugs.webkit.org/show_bug.cgi?id=53326

        * Scripts/webkitpy/common/system/directoryfileset.py:
        * Scripts/webkitpy/common/system/fileset.py:
        * Scripts/webkitpy/common/system/filesystem.py:
        * Scripts/webkitpy/common/system/filesystem_mock.py:
        * Scripts/webkitpy/common/system/filesystem_unittest.py:
        * Scripts/webkitpy/common/system/ospath.py:
        * Scripts/webkitpy/common/system/zipfileset.py:
        * Scripts/webkitpy/common/system/zipfileset_unittest.py:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77093 268f45cc-cd09-0410-ab3c-d52691b4dbfc