[feat] Use pip install --editable in easy_install.develop

Instead of running python setup.py develop directly. This will allow
using zc.buildout.easy_install.develop on recent projects that have
only a pyproject.toml. It also fixes develop leaving build artifacts
in the source directory that caused later runs to do nothing, e.g.
preventing develop-eggs to be rebuilt when a build dependency passed
in setup-eggs option of zc.recipe.egg:develop changed.

A verbosity parameter to tune verbosity of pip is added, with adjusted
values for the case of buildout:develop and of zc.recipe.egg:develop,
so as to remain close to the previous behavior with regards to logs.

Technical details:

For packages using PEP-660-style editable installs, supported by more
recent versions of pip, pip will not delegate to `setup.py develop` -
enabling editable installs for pure pyproject.toml projects - and will
instead generate a .dist-info metadata folder but not a .egg-link.

Since buildout currently requires a .egg-link, as it does not support
PEP 660's mechanism that relies on having a sites-packages directory,
we need to create this .egg-link after the fact. The tricky part is
finding out where the .egg-link should point: the path containing the
pyproject.toml, or a subdirectory?

For this we make use of *.dist-info/top_level.txt to first determine
the list of top-level packages, and then importlib to extract info
from the PEP-660-style install.

If top_level.txt does not exist or is empty, fallback to the path of
the folder that contains the pyproject.toml as the .egg-link target.
If it lists multiple packages that resolve to different import paths,
arbitrarily use the first one and emit a warning.

Support namespace packages where `spec.submodule_search_locations` is
a `_NamespacePath` object instead of a simple `list` and also support
cases where the layout of the source project does not follow the same
structure as the package tree - meaning some custom magic might be
involved in making editable imports work as intended.

src/zc/buildout/buildout.py

@@ -1131,7 +1131,8 @@ class Buildout(DictMixin):
                     for setup in files:
                         self._logger.info("Develop: %r", setup)
                         __doing__ = 'Processing develop directory %r.', setup
-                        zc.buildout.easy_install.develop(setup, dest)
+                        zc.buildout.easy_install.develop(setup, dest,
+                                                         verbosity=-20)
             except:
                 # if we had an error, we need to roll back changes, by
                 # removing any files we created.

src/zc/buildout/tests/easy_install.txt

@@ -348,7 +348,7 @@ reporting that a version was picked automatically:
     zc.buildout.easy_install DEBUG
       Fetching demoneeded 1.1 from: http://.../demoneeded-1.1.zip
     zc.buildout.easy_install DEBUG
-      Running pip install:...
+      Running pip wheel...
     zc.buildout.easy_install INFO
       Got demoneeded 1.1.
     zc.buildout.easy_install DEBUG

src/zc/buildout/tests/test_all.py

@@ -204,8 +204,7 @@ We should be able to deal with setup scripts that aren't setuptools based.
     ... # doctest: +ELLIPSIS
     Installing...
     Develop: '/sample-buildout/foo'
-    ...
-    Installed /sample-buildout/foo
+    Running pip install --editable /sample-buildout/foo
     ...
 
     >>> ls('develop-eggs')
@@ -216,10 +215,9 @@ We should be able to deal with setup scripts that aren't setuptools based.
     ... # doctest: +ELLIPSIS
     Installing...
     Develop: '/sample-buildout/foo'
-    in: '/sample-buildout/foo'
-    ... -q develop -mN -d /sample-buildout/develop-eggs/...
-
-
+    Running pip install --editable /sample-buildout/foo
+    ... -m pip install -t ... --editable /sample-buildout/foo
+    ...
     """
 
 def buildout_error_handling():