cache: use plain dicts for dir cache

[efiop]

See #1820 (comment)

Currently dir cache looks like

[{"relpath": relpath_1, "md5": md5_1}, {"relpath": relpath_2, "md5": md5_2},]

but we could simply store it as

{relpath_1: md5_1, relpath_2: md5_2}

this is obviously going to break backward compatibility, so it is better to do that in 1.0 .

[efiop]

Closing in favor of #829 (comment)