[feat] Let recipe.update access installed paths

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.

src/zc/buildout/buildout.py

@@ -1000,6 +1000,8 @@ class Buildout(DictMixin):
                         part)
 
                 try:
+                    self[part].installed_files = [
+                        p for p in installed_files.split('\n') if p]
                     updated_files = self[part]._call(update)
                 except Exception:
                     installed_parts.remove(part)
@@ -1007,6 +1009,8 @@ class Buildout(DictMixin):
                     installed_part_options['buildout']['parts'] = (
                         ' '.join(installed_parts))
                     raise
+                finally:
+                    del self[part].installed_files
 
                 installed_files = set(installed_files.split('\n')) \
                     if installed_files else set()