Improve root build file and root module detection for Maven reactors.

[siva-sai-udaygiri]

Fixes #1057

What was the problem?

On some Maven reactor (multi-module) projects, the migrator could not find the
root build file and failed with:

RootBuildFileNotFoundException: Could not find BuildFile for root module.

This happened when no BuildFile instance was marked as root, for example
when the top-level pom is not directly at the project root path.

What does this change?

• RootBuildFileFilter

  • Still prefers BuildFile.isRootBuildFile().
  • If no build file is marked as root, it now falls back to the build file
    whose getSourcePath() has the smallest depth (closest to the project root).

• ApplicationModules

  • getRootModule() uses the same fallback: if no module’s build file is
    marked as root, it selects the module whose build file has the shortest
    source path.

• Tests

  • Added RootBuildFileFilterTest.choosesBuildFileWithShortestPathAsRootWhenNoneMarkedAsRoot
    to cover the fallback behaviour with parent/pom.xml and
    parent/module/pom.xml.

Testing

• .\mvnw.cmd -pl components/sbm-core -Dtest=RootBuildFileFilterTest test
• .\mvnw.cmd -pl applications/spring-boot-upgrade -am package -DskipTests
• Ran the built spring-boot-upgrade.jar against a local Maven project to
  verify that the RootBuildFileNotFoundException no longer occurs.