Always look for checkstyle.xml in project root (#3645)

Prefix the checkstyle.xml path with the property
`${maven.multiModuleProjectDirectory}`. This will prevent child modules
from looking inside their own sub-folders for checkstyle.xml. This will
allow us to build child modules from within their root folders without
maven complaining about not being able to find the checkstyle.xml.

Author: lbschanno

pom.xml

@@ -1783,7 +1783,8 @@
                     <artifactId>maven-checkstyle-plugin</artifactId>
                     <version>3.6.0</version>
                     <configuration>
-                        <configLocation>checkstyle.xml</configLocation>
+                        <!-- Use multiModuleProjectDirectory to prevent child modules form looking inside their own sub-folders for checkstyle.xml. -->
+                        <configLocation>${maven.multiModuleProjectDirectory}/checkstyle.xml</configLocation>
                         <failsOnError>true</failsOnError>
                         <failOnViolation>false</failOnViolation>
                         <consoleOutput>true</consoleOutput>