Add support for imports.exclude

[mystor]

This is a fairly straightforward implementation of imports.exclude. The
configuration option is already documented in the book. After changing
this configuration value, you'll need to update imports.lock by running
a successful cargo vet without --locked (or by running cargo vet regenerate imports).

I also added some validation to help catch outdated imports.lock files
when running with --locked. These will never fire when unlocked, as
imports.lock will be automatically updated in those cases.

Fixes #80