Store a part's installed paths in `self[part].installed_files` before
calling `recipe.update()`, to allow the part's recipe to read them if
needed by looking up `options.installed_files`.

Delete this attribute just after the call to ensure this data remains
private to the part.

This will allow zc.recipe.egg:develop to remember in `update()` which
.egg-link it installed in `install()`; inferring this would otherwise
require essentially reinstalling the egg from the setup path, as that
is all the recipe gets as input.

Storing this under `__buildout_installed__` in the part's options was
considered, but some recipes take all the recorded key/value pairs as
input, so it would be a breaking change.