Add enforcer rule to fail when direct dependency <version> is declared

[ctrueden]

All dependency version declarations should be of the form:

<groupId>foo</groupId>
<artifactId>bar</artifactId>
<version>${bar.version}</version>

Otherwise, the melting-pot script, and potentially other tooling like that, will not be able to override the versions at build time.

It is really unintuitive though, so no one knows to do this without being told. Let's let the tooling tell people for us, by failing the build when this situation is detected.