No module can depend on :app (duckduckgo#2135)

Task/Issue URL: https://app.asana.com/0/488551667048375/1202717423070870/f

### Description
Ditto

### Steps to test this PR
- [x] add `implementation project(path: ':app')` to any module that is not the `:app` module
- [x] you should get an error in the IDE or terminal saying that you can't do it

Author: aitorvs

build.gradle

@@ -60,6 +60,10 @@ subprojects {
                             throw new GradleException("Invalid dependency $projectPath -> $dependencyPath. " +
                                     "'api' modules can't depend on other 'api' modules")
                         }
+                        if (dependencyPath.endsWith(":app")) {
+                            throw new GradleException("Invalid dependency $projectPath -> $dependencyPath. " +
+                                    "No other module can depend on ':app'")
+                        }
                     }
                 }
             }