We were using this lock to guarantee a new ID for every file, but we can
get that benefit by using values that are guaranteed to be unique
without the need for locks, such as a UUID.
For example, if `CGFloat` is used in a file where only `UIKit` is imported but not `CoreGraphics`, this will be a violation even if the file previously compiled.
This is because Swift allows referencing some declarations that are only transitively imported.
Enabling the `require_explicit_imports` configuration option will require that the module of every declaration referenced in a source file be explicitly imported.
This will add significant noise to the imports list, but has a few advantages:
1. It will be easier to understand all the dependencies explicitly referenced in a source file.
2. Correcting the `unused_import` rule will no longer introduce compilation errors in files that compiled prior to the correction.
If missing imports are added to a file when correcting it, the `sorted_imports` rule will be automatically run on that file.
If you with to allow some imports to be implicitly importable transitively, you may specify the `allowed_transitive_imports` configuration:
```yaml
unused_import:
require_explicit_imports: true
allowed_transitive_imports:
- module: Foundation
allowed_transitive_imports:
- CoreFoundation
- Darwin
- ObjectiveC
```
JP Simard