component/macros: Add macro.variable.eggs

This is a generalisation of macro.pythonpath.eggs which allows paths
from eggs to be placed into any environment variable, and provides a
way to specify subpaths for each egg from the egg location.

Like macro.pythonpath.eggs, the environment variable is placed in an
environment section at install time after the eggs are installed and
before the environment section is read by the section which uses it.

```
[environment-section]
PATH = some/path

[modify-environment]
<= macro.variable.eggs
variable = ANY_VARIABLE
eggs =
  blue_egg
  yellow_egg
  red_egg
subpaths =
  blue_egg subpath/specific/to/blue
  yellow_egg subpath/specific/to/yellow

[section-that-uses-environment]
recipe = zc.recipe.egg:custom
environment = environment-section
depends =
  ${modify-environment:recipe}

```

component/macros/macro.pythonpath.eggs.cfg

 [buildout]
 parts =
 
-[macro.pythonpath.eggs]
+[macro.variable.eggs]
 recipe = slapos.recipe.build
 init =
   self.eggs = [e.strip() for e in options['eggs'].splitlines() if e.strip()]
 update =
+  import os
   from zc.buildout.easy_install import working_set
   buildout = self.buildout['buildout']
   eggs_directory = buildout['eggs-directory']
   develop_eggs_directory = buildout['develop-eggs-directory']
   dists = working_set(self.eggs, [develop_eggs_directory, eggs_directory])
-  paths = ':'.join(dist.location for dist in dists)
-  self.buildout[options['environment']]['PYTHONPATH'] = paths
-  print("PYTHONPATH=" + paths)
+  subpaths = dict(l.split() for l in options['subpaths'].splitlines())
+  def make_path(dist):
+    subpath = subpaths.get(dist.key)
+    return os.path.join(dist.location, subpath) if subpath else dist.location
+  paths = ':'.join(make_path(dist) for dist in dists)
+  environment = self.buildout[options['environment']]
+  variable = options['variable']
+  value = environment.get(variable)
+  if value:
+    paths = '%s:%s' % (paths, value)
+  environment[variable] = paths
+  print("%s=%s" %(variable, paths))
+variable = PYTHONPATH
+subpaths =
+
+[macro.pythonpath.eggs]
+<= macro.variable.eggs