Fixes: #599

This PR adds a 'rule' configuration option called rules.local_imports. It can be set as usual:

[rules]
local_imports = "error"  # | "warn" | "off"

This rule applies to any Diagnostic that would have been emitted due to an import which is not at the global scope. This includes imports within 'if' statements, functions, classes, or other control structures.

def fun():
    # this is a local import
    from mymodule import api
    api()

The default setting is "error", since today local imports are treated identically to global imports and will produce dependency/interface errors.

Setting local_imports = "off" means that you will no longer see Diagnostics for local imports, even when they violate dependency rules or public interfaces.