Refactor installer code to be more testable

Editor assessment

Refactor installer code to be more testable states a goal, and the doubled refactor in subject and type is redundant. The stated benefit - testability - is real but generic: nothing says what structure changed, which seams were introduced, or what tests become possible. The installer scope is accurate and hygiene is otherwise fine, but a motivation this thin leaves the reader without a lesson. Naming one seam introduced for testing would have substantiated the testability goal that currently stands unexplained.