[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. For this we make use
of importlib to extract info from the PEP-660-style install.

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.

Allow for entries in top_level.txt that are not actually packages,
i.e. where importlib.util.find_spec().submodule_search_locations is
None. A test is added for this case.

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():
@@ -1157,6 +1155,47 @@ existing setup.cfg:
 
 """
 
+def develop_pyproject_only_without_package():
+    """
+
+Install a pyproject-only project that does not contain any package,
+but only a single module.
+
+    >>> mkdir('nopkg')
+    >>> write('nopkg', 'pyproject.toml', '''
+    ... ''')
+
+    >>> write('nopkg', 'xnopkg.py', '''
+    ... print('nopkg')
+    ... ''')
+
+    >>> write('buildout.cfg', '''
+    ... [buildout]
+    ... develop = nopkg
+    ... parts =
+    ... ''')
+
+    >>> print_(system((join('bin', 'buildout'), '-vv')), end='')
+    Develop: '/sample-buildout/nopkg'
+    Found no package import path in develop project /sample-buildout/nopkg for packages ('xnopkg',)
+    Falling back to project's root directory
+
+    >>> ls('develop-eggs')
+    -  xnopkg.egg-link
+    -  zc.recipe.egg.egg-link
+    >>> cat('develop-eggs/xnopkg.egg-link')
+    /sample-buildout/nopkg
+
+"""
+# BBB Python2: skip PEP 660 test
+# Whether the test succeeds actually depends on pip and setuptools versions
+# supporting PEP 660, but Python2 is the reason they would be stuck at old
+# versions, so it will always fail with Python2, and Python3 should use more
+# recent versions.
+if sys.version_info < (3,):
+  del develop_pyproject_only_without_package
+
+
 def uninstall_recipes_used_for_removal():
     r"""
 Uninstall recipes need to be called when a part is removed too: