effective-pom duplicate xml declaration and not pretty printed

### Affected version

3.5.2

### Bug description

The effective-pom goal will generate invalid XML output in some cases. Run 'mvn help:effective-pom -Dverbose=true' on the attached pom.xml and you will see output like that in maven-help-plugin.txt

This output shows the following problems:
* There are two xml declarations, this is actually invalid
* The xml has not been pretty-printed
* Comments from the 'verbose' flag still start with

Start with AbstractEffectiveMojo.prettyFormat() and EffectivePomMojo.execute(), then reproduce the issue using the attached pom.xml and `mvn help:effective-pom -Dverbose=true`. Compare the output with maven-help-plugin.txt; done means the effective POM has one XML declaration, is pretty-printed, and the verbose comments are formatted correctly.

Written by the indexing model from the issue text.