Linkage Monitor unexpectedly recognizes the root parent pom as a BOM

[suztomo]

(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.

[suztomo]

gax-java can generate the artifact list file with a simple task:

task createLinkageMonitorArtifactList {
  doLast {
    new File(projectDir, "linkage-monitor-artifacts.txt").text = """
com.google.api:gax:$version
com.google.api:gax-grpc:$version
com.google.api:gax-httpjson:$version
"""
  }
}