Only rewrite unit files when outdated

[kastiglione]

Unit files are unconditionally imported (read-remap-write), even if they already exist in the destination. index-import could do this conditionally, for example only if the timestamp of the source is newer than the timestamp of the destination. With this feature, index-import could be called twice, with the second time being a no-op.

[brentleyjones]

I think this is addressed by #52?