with -p or -m, error if the package name is not importable

Feature

When the command line is issued specifying scan targets using -p or -m, verify that a package is importable under that name and fail if not.

Pitch

I was scanning two related packages using:
mypy --namespace_packages -p namesp.pkg1 -p namesp_pkg2
The second was a typo, the folder name for the (namespace) package that was a child of $PWD. I got error messages, but they were very confusing, about a file being found in two locations (where the "same" command line, as I thought, had worked properly just two days before). I didn't figure out it was a typo for quite some time.
But obviously, there is no package namesp_pkg2 and an error to that effect would have saved me some time.

Start with mypy's command-line handling for the -p and -m options and reproduce the example with a deliberately misspelled package name. Trace where scan targets are validated, then confirm that an unimportable package produces a direct error instead of confusing duplicate-file diagnostics.