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>