Every pom that does not extend another one (a 'root' pom) has to comply
with the schema ${plugin.resources}/xsd/pom-strict-3.xsd.

Running "pom:validate" on a child pom (a pom that extends another one)
will create another schema ${plugin.resources}/xsd/pom-extend-3.xsd
(if this doesn't exist already) via an xslt transformation using the
stylesheet ${plugin.resources}/xsl/pom-extend-3.xsl. This just replaces
all 'minOccurs="1"' attributes in pom-strict-3.xsd by 'minOccurs="0"'.


Note:

* Every element that is required in a root pom but is not obligatory
  in a child pom, needs a 'minOccurs="1"'.

* Elements that are required in both parent and child poms, must not have
  a 'minOccurs' attribute (so they default to 1).
