Current behavior: if usys.path==["/lib",".flash"] and there's a /lib/foo/ directory, import foo.bar will fail even though help("modules") does list it and .flashstill is inusys.path`.

This is a problem because when I write and debug large programs which don't fit into RAM, I often want to deploy new versions of single modules without disrupting everything else. Right now I have to re-flash the whole board just to add one print() line somewhere. This also raises concerns about the number of write cycles.

Proposed fix: teach MicroPython to check all elements of usys.path for the relevant file-or-directory, instead of stopping at the first one it finds.