Support OS-provided Nim libraries

[FedericoCeratto]

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