Optimize state merging process when there is no base state given

[magodo]

Previously, we were using tfmerge to merge the import state file and the base state file, by a per resource movement manner. This is fine in most cases. But there is still a space to improve the performance, when there is no base state given (i.e. not appending to an existing workspace). In this case, as long as the import items have no address conflicts (guaranteed by aztfy), it can safely merge the state files by simply appending (resources field). Also since this state file is a fresh new one, there is no special requirement on the serial and lineage.

This PR also makes some other less related changes to make it more intuitive for library users:

• Remove the rootdir (i.e. cache dir) creation from NewMeta(). The cache dir is only used to cache an auto-downloaded terraform binary. We now eliminate this behavior due to as a library, this method shall be thread-safe, and we try to avoid downloading behavior as possible. As a result, aztfy now depends on a pre-installed terraform executable that is located at $PATH. Hence mark this PR as a breaking-change.
• Move the import directories creation to meta.Init(), to make it match the tear-down operations in meta.DeInit()
• Config now replaces .Append by .OutputFileNames to be more clear