Editing source/pins.py does not trigger rebuild

Step to reproduce, in a module depending on mbed-hal:

```
yt build
echo 'fail_badly()' >> yotta_module/mbed-hal/source/pins.py
yt build
```

The last command prints "nothing to build" while it should rebuild (and fail, in that case).

Reproduce the issue with the provided `yt build` commands after appending to `yotta_module/mbed-hal/source/pins.py`. Trace how the build determines whether that source has changed, and consider the issue done when the second build detects the edit and rebuilds rather than printing "nothing to build."

Written by the indexing model from the issue text.