[feat] zc.recipe.egg: Reinstall when setup-eggs versions change

Trigger uninstall + install of eggs installed with zc.recipe.egg:custom
or :develop when pinned versions of setup-eggs have changed. To achieve
this the versions of setup-eggs are included in the section: this makes
them part of its signature so that when they change, buildout will call
`uninstall` and `install` for this section instead of just `update`.

Unlike other zc.recipe.egg entry points, :custom stores the path of the
installed egg; thus `uninstall` will remove it fully, leaving `install`
to reinstall it cleanly from scrach.

In the case of :develop, `uninstall` matters little as only the path of
the installed `.egg-link` is stored. Instead `install` must be fixed to
actually rebuild the egg in-place in the source directory and `develop`
should do nothing.

The main issue lies in `zc.buildout.easy_install.develop`: depending on
the build process, it may leave build artifacts in the source directory
that cause future runs to do nothing.

zc.recipe.egg_/src/zc/recipe/egg/custom.py

@@ -59,6 +59,16 @@ class Base:
 
         self.newest = buildout['buildout'].get('newest') == 'true'
 
+        self.setup_eggs = setup_eggs = [
+            r.strip()
+            for r in options.get('setup-eggs', '').split('\n')
+            if r.strip()]
+
+        # Put setup_eggs versions in section to include them in signature,
+        # so that changing them will trigger uninstall + install.
+        options['_v'] = ' '.join(
+            buildout.versions.get(egg, 'none') for egg in setup_eggs)
+
 
 
     def install(self):
@@ -94,10 +104,7 @@ class Base:
 
     def _install_setup_eggs(self):
         options = self.options
-        setup_eggs = [
-            r.strip()
-            for r in options.get('setup-eggs', '').split('\n')
-            if r.strip()]
+        setup_eggs = self.setup_eggs
         if setup_eggs:
             ws = zc.buildout.easy_install.install(
                 setup_eggs, options['_e'],
@@ -177,6 +184,11 @@ class Develop(Base):
         return zc.buildout.easy_install.develop(
             options['setup'], options['_d'], self.build_ext)
 
+    def update(self):
+        # If any option changes or the source directory is reinstalled
+        # or the .egg-link is deleted, install will be called instead.
+        pass
+
 
 def build_ext(buildout, options):
     result = {}