[MPH-168] effective pom should support multi-module project

**[Or Shachar](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=orshachar)** opened **[MPH-168](https://issues.apache.org/jira/browse/MPH-168?redirect=false)** and commented

I need to get effective pom of all modules in my multi-module project. Today this requires me to discover all modules manually and run effective pom on each of them.

For multi-module projects it would be great if I can run effective pom from top level and get an effective pom written to `${project.build.directory}/effective-pom.xml` of each module. 

Since effective-pom does not write to file by default maybe a reasonable usage would be to run:

```java
mvn help:effective-pom -Doutput=target/effective.pom.xml
```

Maybe helpful: mvn dependency plugin lets you do it (with goals "list" or "collect").

---

**Affects:** 3.2.0

**Remote Links:**
- [GitHub Pull Request #76
](https://github.com/apache/maven-help-plugin/pull/76)