Enforce consistent way of referring to crate root

What it does

There are a few different ways by which to refer to the root of a crate:

• crate
• super
• Crate name

This lint enforces a consistent way of referring to the root of a crate.

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I don't think there is much consensus in the community on any one of these styles being better than the other but it's nice for the one style to be adhered to throughout a crate. I figure this belongs in restriction.

Currently there are some overlapping lints in restriction eg. implicit_return/needless_return. The user selects a particular style by enabling one and disabling the other.

Perhaps it makes sense to do the same thing here and so this lint suggestion is really a suggestion for 3 new lints.

Advantage

Makes searching code easier. If I want to find all places where root of crate is referenced, I can find them easier.

Makes some kinds of refactoring easier. If I want to nest some existing modules inside a new module, it is marginally easier if there is a single kind of root reference to update.

Drawbacks

It might make you use a particular way of referring to crate root that isn't appropriate for the instance

If a crate has a particularly long name

use crate::submodule;

could become

use long_crate_name_that_is_painful_to_write_and_takes_time_to_say::submodule;

Example

Treating the different styles separately:

crate

use super::module_a;
use crate_name::module_b;

Could be written as:

use crate::module_a;
use crate::module_b;

super

use crate::module_a;
use crate_name::module_b;

Could be written as:

use super::module_a;
use super::module_b;

Crate name

use crate::module_a;
use super::module_b;

Could be written as:

use crate_name::module_a;
use crate_name::module_b;

Start by reviewing the issue discussion and the existing restriction lints implicit_return and needless_return, which are cited as a possible model. Resolve whether this should be one lint or three and which crate-root style each should enforce; done requires an agreed design and corresponding implementation and tests.