maven: Discard templated origin URLs

While this doesn't fix the actual bug in swh-loader-svn#4729 , it prevents this particular origin from being listed in the first place.

softwareheritage-scheduler=> \copy (select url from listed_origins inner join listers on (listers.id=lister_id) where listers.name = 'maven' and url like '%$%') to stdout
${identityconnectors.svn.trunk.projects}
${maven.scm.anonymous}@wicket-stuff.svn.sourceforge.net:/svnroot/wicket-stuff/trunk/wicketstuff-core/jasperreports-parent
${scm.repo.readonly.url}
${staccatissimo.scm.base}/trunk
${staccatissimo.scm.base}/trunk/commons-parent-pom
a${scm.url}
file:///${basedir}
git://forge.fusesource.com/${forge-project-id}.git
http://${scm.dir}
http://json-smart.googlecode.com/svn/branches/${version}/
http://svn.apache.org/repos/asf/lucene/dev/trunk/${module-directory}
https://${java.net.username}@glassfish-svn.dev.java.net/glassfish-svn/gfsvn/trunk/v3/${pom.artifactId}
https://${java.net.username}@glassfish-svn.dev.java.net/glassfish-svn/trunk/api/
https://jamon.svn.sourceforge.net/svnroot/jamon/trunk/jamon-eclipse/${project.artifactId}/
https://jamon.svn.sourceforge.net/svnroot/jamon/trunk/jamon/${artifactId}/
https://svn.apache.org/repos/asf/tapestry/tapestry5/${pom.artifactId}/trunk/
https://svn.sourceforge.net/svnroot/wicket/branches/WICKET_1_2/${project.artifactId}
https://www.ja-sig.org/svn${scm.path}
pserver:anonymous@cvs.sourceforge.net:/cvsroot/${pom.artifactId}:JPOX
svn+ssh://${maven.username}@svn.forge.objectweb.org/svnroot/jonas/jonas/tags/JONAS_5_2_0_M2_SR1
svn+ssh://source.israfil.net/repos/svn/mojo/trunk/${project.artifactId}

Example POMs involved in generating these URLs:

• https://repo1.maven.org/maven2/org/ow2/jonas/jonas/5.2.0/jonas-5.2.0.pom
• https://repo1.maven.org/maven2/wicket/wicket/1.2/wicket-1.2.pom