fix: allow multiple --location-prefix-rewrite command-line options

[maciekwawro]

It seems that it was the intention to begin with: https://github.com/pabigot/pyxb/blob/next/pyxb/binding/generate.py#L2569-L2570

Test plan

Before:

$ ls
schema.xsd
$ pyxbgen --schema-location=/A/schema.xsd --location-prefix-rewrite=/A=./ --location-prefix-rewrite=/B=./
ERROR:pyxb.utils.utility:open /A/schema.xsd
IOError: [Errno 2] No such file or directory: '/A/schema.xsd'

After:

$ pyxbgen --schema-location=/A/schema.xsd --location-prefix-rewrite=/A=./ --location-prefix-rewrite=/B=./
Python for http://www.example.com/ requires 1 modules

I also confirmed that $ pyxbgen --help still works.