Previously, the Maven plugin integration tests used a settings.xml file
that defined https://repo.spring.io/snapshot as a repository. This
allowed them to resolve snapshots of the plugin's Spring Framework
dependencies but it had the unfortunate side-effect of also allowing
them to resolve snapshots of other Spring Boot modules from Artifactory
rather than using those currently being built.

This commit replaces the repositories in settings.xml with a Gradle
task that resolves the necessary dependencies and populates a local
repository with the dependencies' jars and pom files. This is achieved
using a ComponentMetadataRule that creates a custom variant of each
dependency that includes its pom file, inspired by the example in
gradle/gradle/#11449. A configuration that extends the
runtimeClasspath configuration and select the custom variant via its
attribute is then used to resolve the jars and pom files of the runtime
classpath such that they can then be used to populate the local
repository.

Closes gh-22828