Refinement of Providers into Providers and ExternProviders

[xizheyin]

Refined some vague concepts and pseudo-code in Query System

In the past, we only used vague concepts like local and extern to differentiate between different queries. however the code has more detailed Providers and ExternProviders to differentiate between them, so we should mention both of them in the documentation as well.

It is also important to indicate that rustc_middle::util::Providers is made up of Providers and ExternProviders in the query mod.

This PR also updates the example of registering a new Provider to consider ExternProviders.

r? @tshepang

[xizheyin]

first commit for ExternProviders, second commit for the nit in #2465.