Move _collectfile to FSCollector (#6830)

Previously this was implemented both on `Session` and `Package`, where
the extra code in `Package._collectfile` was not covered/used.

Ref: https://github.com/pytest-dev/pytest/pull/6830#issuecomment-592663236

src/_pytest/main.py

@@ -587,28 +587,6 @@ class Session(nodes.FSCollector):
                 return
             yield from m
 
-    def _collectfile(self, path, handle_dupes=True):
-        assert (
-            path.isfile()
-        ), "{!r} is not a file (isdir={!r}, exists={!r}, islink={!r})".format(
-            path, path.isdir(), path.exists(), path.islink()
-        )
-        ihook = self.gethookproxy(path)
-        if not self.isinitpath(path):
-            if ihook.pytest_ignore_collect(path=path, config=self.config):
-                return ()
-
-        if handle_dupes:
-            keepduplicates = self.config.getoption("keepduplicates")
-            if not keepduplicates:
-                duplicate_paths = self.config.pluginmanager._duplicatepaths
-                if path in duplicate_paths:
-                    return ()
-                else:
-                    duplicate_paths.add(path)
-
-        return ihook.pytest_collect_file(path=path, parent=self)
-
     @staticmethod
     def _visit_filter(f):
         return f.check(file=1)

src/_pytest/nodes.py

@@ -502,6 +502,28 @@ class FSCollector(Collector):
         ihook.pytest_collect_directory(path=dirpath, parent=self)
         return True
 
+    def _collectfile(self, path, handle_dupes=True):
+        assert (
+            path.isfile()
+        ), "{!r} is not a file (isdir={!r}, exists={!r}, islink={!r})".format(
+            path, path.isdir(), path.exists(), path.islink()
+        )
+        ihook = self.gethookproxy(path)
+        if not self.isinitpath(path):
+            if ihook.pytest_ignore_collect(path=path, config=self.config):
+                return ()
+
+        if handle_dupes:
+            keepduplicates = self.config.getoption("keepduplicates")
+            if not keepduplicates:
+                duplicate_paths = self.config.pluginmanager._duplicatepaths
+                if path in duplicate_paths:
+                    return ()
+                else:
+                    duplicate_paths.add(path)
+
+        return ihook.pytest_collect_file(path=path, parent=self)
+
 
 class File(FSCollector):
     """ base class for collecting tests from a file. """

src/_pytest/python.py

@@ -593,31 +593,6 @@ class Package(Module):
     def gethookproxy(self, fspath: py.path.local):
         return super()._gethookproxy(fspath)
 
-    def _collectfile(self, path, handle_dupes=True):
-        assert (
-            path.isfile()
-        ), "{!r} is not a file (isdir={!r}, exists={!r}, islink={!r})".format(
-            path, path.isdir(), path.exists(), path.islink()
-        )
-        ihook = self.gethookproxy(path)
-        if not self.isinitpath(path):
-            if ihook.pytest_ignore_collect(path=path, config=self.config):
-                return ()
-
-        if handle_dupes:
-            keepduplicates = self.config.getoption("keepduplicates")
-            if not keepduplicates:
-                duplicate_paths = self.config.pluginmanager._duplicatepaths
-                if path in duplicate_paths:
-                    return ()
-                else:
-                    duplicate_paths.add(path)
-
-        if self.fspath == path:  # __init__.py
-            return [self]
-
-        return ihook.pytest_collect_file(path=path, parent=self)
-
     def isinitpath(self, path):
         return path in self.session._initialpaths