Fixes bug 372: Rename refactoring should use workspace.validateFiles.

The rename refactorer in PyRenameEntryPoint does not currently
validate its changes.  To cooperate well with RepositoryProviders,
this patch adds the necessary validation step and centralizes the
logic in ChangedFilesChecker to be used by both the ReplaceRefactoring
and PyRenameEntryPoint.

Change-Id: I7d5b1f8f59206e248a1810ab38305c7f0cc2b921