[MCHECKSTYLE-397] support ${config_loc} on checkstyle xml

[qxo]

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

with this PR, we can use same checkstyle.xml which contains ${config_loc}

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

https://issues.apache.org/jira/projects/MCHECKSTYLE/issues/MCHECKSTYLE-397

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[qxo]

mvn -Prun-its clean verify failed on my PC because my network issue:

javax.net.ssl.SSLException: Connection reset
    at sun.security.ssl.Alert.createSSLException (Alert.java:127)
    at sun.security.ssl.TransportContext.fatal (TransportContext.java:321)
    at sun.security.ssl.TransportContext.fatal (TransportContext.java:264)
    at sun.security.ssl.TransportContext.fatal (TransportContext.java:259)
    at sun.security.ssl.SSLTransport.decode (SSLTransport.java:137)
    at sun.security.ssl.SSLSocketImpl.decode (SSLSocketImpl.java:1152)

[elharo]

This could use some tests.

[lrozenblyum]

@bmarwell @elharo it's really a nice feature to have in the plug-in.
Currently enabling suppressions for a Maven multi-module project is much more complex than it could be.
${config_loc} support should enable seamless experience for Checkstyle users.

So could I for example continue work on this feature if it has chances for merge?
As I understand this pull request remarks should be solved?