Continuing to shave small bits of time off startup --
`spack.cmd.common.arguments` constructs many `Args` objects at module
scope, which has to be done for all commands that import it. Instead of
doing this at load time, do it lazily.
- [x] construct Args objects lazily
- [x] remove the module-scoped argparse fixture
- [x] make the mock config scope set dirty to False by default (like the
regular scope)
This *seems* to reduce load time slightly
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