instrumentation: extract duplication to get list of installed files

instrumentation: extract duplication to get list of installed files

Before and after the building of each package, the instrumentation hooks are
run. One of these hooks obtains the list of files installed by a package.

The code to obtain this list is currently duplicated in the start and end
part of the hook. While the amount of duplication is currently small, a
subsequent patch will make more changes to this code, increasing the
duplication.

Therefore, split off into a helper function.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>