[MNG-6830] Warn on missing namespace

**[Elliotte Rusty Harold](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=elharo)** opened **[MNG-6830](https://issues.apache.org/jira/browse/MNG-6830?redirect=false)** and commented

We allow pom.xml files with no namespace and reject pom.xml files that use prefixed local names that are correctly namespaced.

As step 1 to getting to a release where we can fix this, emit a warning when we encounter a non-namespaced pom.xml such as this:

\
\4.0.0\
\war\
\0.1.0-SNAPSHOT\

\c.g\
\mvn25\

\
\2.2.0\
\UTF-8\
\UTF-8\
\1.8\
\1.8\
\true\
\
\

---
No further details from [MNG-6830](https://issues.apache.org/jira/browse/MNG-6830?redirect=false)

No files or tests are named; start by locating Maven core's POM XML parsing and namespace validation entry points. Use the non-namespaced pom.xml example as a test case, and consider the work done when processing it emits a warning without changing the existing behavior.

Written by the indexing model from the issue text.