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This splits the manifest file loading logic from makemanifest.py and
updates makemanifest.py to use it.

This will allow non-freezing uses of manifests, such as defining packages
and dependencies in micropython-lib.

Also adds additional methods to the manifest "API":
 - require() - to get a package from micropython-lib.
 - module() - to define a single-file module
 - package() - to define a multi-file package

module() and package() should replace most uses of freeze() and can also
be also used in non-freezing scenarios.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>