Support OS-provided Nim libraries
Many OSes provide a package manager that can install libraries regardless of their language.
Similarly to Python, Ruby, Rust, Go, C, Perl and so on, Nim and Nimble should be able to locate OS-provided libraries and use them.

Contrarily to #80 this feature is needed only for OS-installed libraries and requires Nim/Nimble to:
- find packages installed globally based on a hardcoded path. The path is set by the OS package maintainers.
- don't expect to write in such location
- don't attempt to find packages elsewhere or do networking unless specified otherwise
