Resolve versionManagement configuration lazily and preserve exclusions

Previously, the versionManagement configuration was resolved as part of
the Boot Gradle plugin being applied. This meant that no dependencies
could be added to it and attempting to do so would result in a failure:
“You can't change a configuration which is not in unresolved state”.
This commit updates ApplyExcludeRules to wrap its processing in a
before resolve action. This defers the resolution of the
versionManagement configuration until one of the project’s other
configurations is being resolved. Fixes #1077

In addition to the above, the transitive exclusions that the Gradle
plugin provides were being lost if custom version management provided
a version for the same dependency. This commit updates
AbstractDependencies to preserve the exclusions from an existing
dependency declaration while using the version from the newer
dependency. This ensures that the exclusions remain while allowing
versions to be overridden. Fixes #1079