Accept both tarballs and pure-python wheels as sources

This redoes 0abf873d (Install egg from wheel only if explictly required),
because that switch to install wheels only when explicitly requested
broke things in several places[1,2]. We can avoid such breakage if
instead of rejecting all wheels by default, we accept wheels, but allow
only pure-python wheels to be installed.

As nexedi/slapos!1026 (comment 138639)
shows detecting whether a dist is pure-python wheel is just

	(dist.precedence == WHL_DIST and dist.platform is None)

So let's use this and rework egg installer accordingly.

This patch is completely untested. However the way to check whether a
wheel is source-only was tested at nexedi/slapos!1026 (comment 138639).

If this patch is accepted and new slapos.buildout release is made, then
all :whl usage in slapos.git can be dropped.

/cc @tomo, @jerome, @jm

[1] nexedi/slapos!1026
[2] nexedi/slapos!1026 (comment 138590)

src/zc/buildout/easy_install.py

@@ -561,12 +561,13 @@ class Installer:
             return None
 
         # Filter the available dists for the requirement and source flag
-        wheel = getattr(requirement, 'wheel', False)
+        # Accept tarballs and pure-python wheels as sources
         dists = [dist for dist in index[requirement.project_name]
                  if ((dist in requirement)
-                     and (dist.precedence == WHL_DIST) == wheel and
+                     and
                      ((not source) or
-                      (dist.precedence == pkg_resources.SOURCE_DIST)
+                      ((dist.precedence == pkg_resources.SOURCE_DIST) or
+                       (dist.precedence == WHL_DIST and dist.platform is None))
                       )
                      )
                  ]
@@ -786,6 +787,9 @@ class Installer:
         """Return requirement with optional [versions] constraint added."""
         constraint = self._versions.get(requirement.project_name.lower())
         if constraint:
+            # accept `<egg>:whl` as just `<egg>` for backward compatibility
+            # because once we were handling :whl suffix as explicit request to
+            # install via wheel and parts of slapos still use this.
             wheel = constraint.endswith(':whl')
             if wheel:
                 constraint = constraint[:-4]
@@ -796,8 +800,6 @@ class Installer:
                 logger.info(self._version_conflict_information(
                     requirement.project_name.lower()))
                 raise
-            if wheel:
-                requirement.wheel = True
 
         return requirement