Move existing API to `api.v4` package

• Copy all API classes across to api.v4
• Make a copy of the compatibility tests to test the classes in the api.v4 package.
• Rework all the original classes that is mentioned below to forward to the new package.
• Mark all existing classes that is mentioned below as deprecated.
• Move all classes in internal package to internal.v4.
• Update the documentation with a note about the package change and the strategy why the change is occurring. Also note that the old packages will be removed probably in the 1.0 release or shortly after.
• Replace of references to groovy doc in the documentation to point to the api.v4 package.

Classes to remain at api level:

• Everything in errors package
• BaseProgressLogger.groovy (but convert to Java interface)
• CheckSumVerification.groovy (but convert to Java interface)
• ClassLocation.java
• ExclusiveFileAccess.groovy
• OperatingSystem.groovy and everything in the os package
• ProgressLogger.groovy (but convert to Java interface)
• Version.groovy

Classes to deprecate:

• AbstractCombinedProjectTaskExtension.groovy
• AbstractDistributionInstaller.groovy
• artifacts.ArtifactAttributeContainer.groovy
• artifacts.DependencyCache.groovy
• ClosureUtils.groovy
• exec.AbstractCommandExecSpec.groovy
• exec.AbstractCommandExecTask.groovy
• exec.AbstractExecSpec.groovy
• exec.AbstractExecTask.groovy
• exec.AbstractExecWrapperTask.groovy
• exec.AbstractScriptExecSpec.groovy
• exec.AbstractScriptExecTask.groovy
• exec.AbstractToolExecSpec.groovy
• exec.AbstractToolExtension.groovy
• exec.ExecSpecInstantiator.groovy
• exec.ExternalExecutable.groovy
• exec.ExternalExecutableType.groovy
• exec.NamedResolvedExecutableFactory.groovy
• exec.ResolvableExecutable.groovy
• exec.ResolvableExecutableType.groovy
• exec.ResolvedExecutableFactory.groovy
• exec.ResolvedExecutable.groovy
• exec.ResolveExecutableByVersion.groovy
• exec.ResolverFactoryRegistry.groovy
• ExtensionUtils.groovy
• FileUtils.groovy
• git.AbstractCloudGit.groovy
• git.CloudGitConfigurator.groovy
• git.CloudGitDescriptor.groovy
• git.GitHubArchive.groovy
• git.GitLabArchive.groovy
• git.GitRepoArchiveDownloader.groovy
• JavaForkOptions.groovy
• LogUtils.groovy
• MapUtils.groovy
• PropertyResolveOrder.java
• PropertyResolver.groovy
• repositories.AuthenticationSupportedRepository.groovy
• repositories.SimplePasswordCredentials.groovy
• StringUtils.groovy
• TaskProvider.groovy
• UnpackUtils.groovy
• UriUtils.groovy
• wrapper.script.AbstractCacheBinaryTask.groovy
• wrapper.script.AbstractScriptWrapperTask.groovy