(This is a regression of #1920)

The problem in java-asset repository https://github.com/googleapis/java-asset/issues/570 was caused by Linkage Monitor unexpectedly recognizing the root pom.xml as a BOM and thus reading the dependencyManagement section to create a snapshot Libraries BOM.

We want to read the dependencyManagement section of BOMs to populate LinkageMonitor.localArtifacts, because gax-java repository has gax-bom to tell which artifacts are generated by the repository.

Note that google-cloud-bom is a BOM but LinkageMonitor.localArtifacts does not need to have its dependencyManagement section.