Look in debian/tmp for files to move

Sven Joachimrequested to merge
joachim-guest/dh-exec:dh-exec-install-move-fix into master

Commit ef2b8514 introduced a new feature where filenames prefixed with => would be moved rather than copied from the source to the destination. Unfortunately it would fail when the files are located in debian/tmp, which is the most common use case for this feature.

Also add a new test to ensure that this feature works.

Closes: #1028347

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