Jim Mussared a7fa18c203 py/builtinimport: Refactor module importing.
Simplify and document/comment the handling of builtin import for:
- already-loaded modules
- built-in modules
- built-in umodules (formerly weak links)
- filesystem modules

Retains existing functionality with smaller code size but should also
facilitate potential new features (built-in packages, controlling the
frozen path).

Also makes the (unix-only) -m behavior a bit more obvious and configurable.

Code size change with this commit:

   bare-arm:    +0 +0.000%
minimal x86:   -64 -0.039%
   unix x64:   -32 -0.006%
unix nanbox:    -4 -0.001%
      stm32:  -184 -0.047% PYBV10
     cc3200:  -120 -0.065%
    esp8266:  -228 -0.033% GENERIC
      esp32:  -268 -0.018% GENERIC[incl +16(data)]
        nrf:  -152 -0.087% pca10040
        rp2:  -256 -0.052% PICO
       samd:   -80 -0.057% ADAFRUIT_ITSYBITSY_M4_EXPRESS
2021-12-01 13:23:34 +11:00
..
2021-09-19 23:35:10 +10:00
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2021-09-19 23:35:10 +10:00
2019-11-21 12:04:53 +11:00
2020-10-22 11:40:56 +02:00
2020-10-22 11:40:56 +02:00