glapi: Avoid argparse type argument for API XML input files.

glapi: Avoid argparse type argument for API XML input files.

argparse type is a nice type saver for simple data types, but it doesn't
look a good fit for the input XML file:

- Certain implementations of argparse (particularly python 2.7.3's)
  invoke the type constructor for the default argument even when an
  option is passed in the command line.  Causing `No such file or
  directory: 'gl_API.xml'` when the current dir is not
  src/mapi/glapi/gen.

- The parser takes multiple arguments.  This is currently worked around
  using lambdas, but that unnecessarily complex and hard to read.
  Furthermore it's odd to have a side-effect as heavy as parsing XML
  happening deep inside the argument parsing.

https://bugs.freedesktop.org/show_bug.cgi?id=90600

Reviewed-by: Brian Paul <brianp@vmware.com>