py/builtinimport: Handle empty sys.path correctly.

If sys.path is enabled, but empty, this will now no longer search the
filesystem. Previously an empty sys.path was equivalent to having
`sys.path=[""]`. This is a breaking change, but this behavior now matches
CPython.

This also provides an alternative mechanism to the u-prefix to force an
import of a builtin module:

```
import sys
_path = sys.path[:]
sys.path.clear()
import foo  # Forces the built-in foo.
sys.path.extend(_path)
del _path
```

Code size diff is -32 bytes on PYBV11.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

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