keg: factor out dylib id calculation
keg: decouple dylib id changes from install name changes

The dylib id calculation does not use any information gathered in
install_names_for, so we can pull the logic out completely and only
yield the install names.
keg: yield install names successively instead of all at once

Every caller of install_names_for loops over the yielded array. Now that
we have separated the dylib id calculation and update, we can eliminate
the need for a separate loop and yield each install name individually.

Rename this method to "each_install_name_for" for clarity.