mip: Make mip.install() skip /rom*/lib directories.

[dpgeorge]

If a ROMFS is mounted then "/rom/lib" is usually in sys.path before the writable filesystem's "lib" entry. The ROMFS directory cannot be installed to, so skip it if found.

Tested on PYBD_SF2.

Prior to this PR:

>>> import mip
>>> mip.install('unittest')
Installing unittest (latest) from https://micropython.org/pi/v2 to /rom/lib
Copying: /rom/lib/unittest/__init__.mpy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "mip/__init__.py", line 1, in install
  File "mip/__init__.py", line 1, in _install_package
  File "mip/__init__.py", line 1, in _install_json
  File "mip/__init__.py", line 1, in _download_file
  File "mip/__init__.py", line 1, in _ensure_path_exists
AttributeError: 'VfsRom' object has no attribute 'mkdir'

With this PR applied:

>>> import sys
>>> sys.path
['', '.frozen', '/rom', '/rom/lib', '/flash', '/flash/lib']
>>> import mip
>>> mip.install('unittest')
Installing unittest (latest) from https://micropython.org/pi/v2 to /flash/lib
Exists: /flash/lib/unittest/__init__.mpy
Done