malformed external_dependencies (edge-case)

I have a CirPy lib that uses `spidev` on Linux (yields much better performance than pureio). I have listed spidev in the lib's requirements.txt like so
```txt
adafruit-circuitpython-busdevice
spidev; sys_platform == 'linux'
```
The `; sys_platform == 'linux'` is advice taken from pip's docs, but this makes the bundle's JSON external_dependencies look like so:
```json
"external_dependencies": [
"adafruit-circuitpython-busdevice",
"spidev; sys_platform "
]
```

I'm mandating the spidev module on Linux only because linting the lib when developing requires that the development changes be installed (spidev can't be installed on Windows). Granted, I still have to ignore warnings about `import spidev` in my lib's examples, but that's what hardware testing is for.

Feel free to close this issue if there are no foreseen consequences with circuitpython.org.

Trace the requirements.txt parsing that produces the external_dependencies JSON, using the issue's spidev entry with its sys_platform marker as the reproduction case. Confirm what the generated dependency should retain, then verify the corrected output against the bundle-generation behavior; the payload names no source file or test.

Written by the indexing model from the issue text.