Add support for `mpremote mip install package.json` where `package.json` is
a json file on the local filesystem.
Without this, package json files can only be loaded from http, https,
github or gitlab URLs.
This is useful for testing `package.json` files for pacages in development
and for constructing one's own `package.json` files for Python packages
which are not yet available for installation using mip.
Signed-off-by: Glenn Moloney <glenn.moloney@gmail.com>