[MCHECKSTYLE-397] support ${config_loc} in checkstyle.xml just like eclipsecs

**[qxo](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=qxo)** opened **[MCHECKSTYLE-397](https://issues.apache.org/jira/browse/MCHECKSTYLE-397?redirect=false)** and commented

we can use config_loc as: ${config_loc}/suppressions.xml.

so we can use same checkstyle.xml which contains ${config_loc} for eclipsecs and maven

for eclipsecs ${config_loc} is a variable of Eclipse and it will refer to the same directory where the checkstyle.xml is present.

ref: https://checkstyle.org/eclipse-cs/#!/properties

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**Remote Links:**
- [GitHub Pull Request #36
](https://github.com/apache/maven-checkstyle-plugin/pull/36)

Start by reviewing how the plugin loads checkstyle.xml and compare the requested ${config_loc} behavior with EclipseCS. Inspect the referenced GitHub Pull Request #36 and the suppressions.xml example; done means the same checkstyle.xml works in both EclipseCS and Maven.

Written by the indexing model from the issue text.