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This changes makemanifest.py & mpy-tool.py to merge string and mpy names
into the same list (now mp_frozen_names).

The various paths for loading a frozen module (mp_find_frozen_module) and
checking existence of a frozen module (mp_frozen_stat) use a common
function that searches this list.

In addition, the frozen lookup will now only take place if the path starts
with ".frozen", which needs to be added to sys.path.

This fixes issues #1804, #2322, #3509, #6419.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>