dry_system provider sources should be lazy-loadable

[solnic]

Currently we have dry/system/provider_sources file which eager-loads all files from lib/dry/system/provider_sources/* as new providers under :dry_system key.

This introduces a problem from the usage point of view because you need to remember to require that file. It would be better to just make it lazy-loadable instead, so that if you try to use a dry_system provider, it just knows how to lazy load it.

For context: dry-rb/dry-rails#55 (comment)