py/builtinimport: Add compile-time option to disable external imports.
The new option is MICROPY_ENABLE_EXTERNAL_IMPORT and is enabled by default so that the default behaviour is the same as before. With it disabled import is only supported for built-in modules, not for external files nor frozen modules. This allows to support targets that have no filesystem of any kind and that only have access to pre-supplied built-in modules implemented natively.
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