The reason task should list the classes which depend on a dependency

**Is your feature request related to a problem? Please describe.**
The below is a shorted snippet of the reason task
```
gw category-a:subproject:reason --id :category-b:subproject

Source: main
------------
* Uses class com.company.my.special.class (implies implementation).
```
While knowing which class is used is a good start, knowing **where** it is used is key to verify/remove a dependency.

**Describe the solution you'd like**
Output in line with the following would be nice to have:
```
gw category-a:subproject:reason --id :category-b:subproject

Source: main
------------
* Uses class com.company.my.special.class (implies implementation), listing first X occurrences:
- FQClass1
- FQClass2
```
This information is available in the `synthetic-project.json` and other intermediates.

Start by tracing the reason task that produces the shown `reason` output and inspect `synthetic-project.json` and the other intermediates for the available class-usage data. Done means the dependency explanation lists the requested first X dependent classes in the demonstrated format, with appropriate coverage for the existing reason output.

Written by the indexing model from the issue text.