OrderedImports should create separate groups for different access levels

[allevato]

Now that public/internal/private/etc import is supported, we should have the OrderedImports rule do separate groupings for each access level. Order the groups in descending order of access level (public, package, internal, fileprivate/private), with each group sorted lexicographically as usual.

Other details:

• If a user has both internal import and import, do we unify them by adding/removing internal? We don't currently add or drop internal anywhere else, so we don't have a precedent here.
• We do need to respect the fileprivate/private configuration that's used for other file-scoped declarations.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://126948323

[ebgraham]

import grouping seems to already exist for @testable imports, can it be extended to include @_implementationOnly?

[allevato]

If we're planning to do it for internal/private/etc imports, doing it also for @_implementationOnly seems reasonable too.

[ebgraham]

SE-0409 adds an upcoming flag for a bare import to be internal, but this likely won't be enabled until Swift 7. So we probably need an OrderedImports config option on how to group bare imports. I also think fileprivate/private and omitting a modifier should be a separate config (or rule).

Below is one potential grouping with bare imports. Putting it between public and internal is because it can be either depending on the default. Package throws a wrench into that, but package isn't particularly common.

public import A
public import B

import C
import D

internal import G
internal import H

package import E
package import F

fileprivate import I
fileprivate import J

private import K
private import L

@_implementationOnly import M
@_implementationOnly import N

@testable import O
@testable import P

Another option is to move bare imports together with their modifier prefixed ones, based on a whether public/internal is the default (specified as a rule config):

import A
public import B
public import C
import D

or

import A
internal import B
internal import C
import D

Another option is to always put bare imports at the top.

How does this interact in combination with declaration imports? We probably need to remove the grouping for those.

Should there be spaces between groups? I think the spaces make it hard to read as the number of groups increases.