[MCHECKSTYLE-355] Allow checkstyleRules next to configLocation

**[HerrDerb](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=herrderb)** opened **[MCHECKSTYLE-355](https://issues.apache.org/jira/browse/MCHECKSTYLE-355?redirect=false)** and commented

**Summary**

Allow to define a `configLocation` and `checkstyleRules` resulting in inline checkstyle rules overriding `configLocation` checks. This would allow a dynamic checkstyle rule configuration.

**Current behaviour**
Currently when running the plugin with the following configuration

```java

       google_checks.xml
      
              
                     
             
      
 
```

 it fails and returns

```java
Failed to execute goal org.apache.maven.plugins:maven-checkstyle-plugin:3.0.0:check (default) on project comlab-pom: If you use inline configuration for rules, don't specify a configLocation
```

**Desired behaviour**

The google check `LineLenth` is overriden by the inline check, defining a valid line length of 120 instead of 80.

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No further details from [MCHECKSTYLE-355](https://issues.apache.org/jira/browse/MCHECKSTYLE-355?redirect=false)

Start by reproducing the reported Maven configuration with configLocation and checkstyleRules, then trace how the plugin handles these two settings. Identify the existing validation or configuration-merging entry point; done means inline rules override the corresponding configLocation rule without the current failure, with coverage for the LineLength example.

Written by the indexing model from the issue text.