Consider parallel execution in P2ResolverImpl.resolveTargetDependencies

[laeubi]

P2ResolverImpl contains a loop to resolve for different TargetEnvironment in

org.eclipse.tycho.p2.resolver.P2ResolverImpl.resolveTargetDependencies(TargetPlatform, ReactorProject)

for (TargetEnvironment environment : environments) {
    if (isMatchingEnv(metadata, environment, logger::debug)) {
        results.put(environment, resolveDependencies(Collections.<IInstallableUnit> emptySet(), project,
                new ProjectorResolutionStrategy(logger), environment, targetPlatform));
    } else {
        logger.info(MessageFormat.format(
                "Project {0}:{1}:{2} does not match environment {3} skipp dependecy resolution",
                project.getGroupId(), project.getArtifactId(), project.getVersion(),
                environment.toFilterExpression()));
    }
}

we should consider if it is possible to resolve each platform in a separate thread,