main: improve message on `pytest path/to/a/directory::mytest`

The path part of a `<path>::part1::part2` style collection argument must
be a file, not a directory.

Previously this crashed with an uncool assert "invalid arg".

src/_pytest/main.py

@@ -808,6 +808,7 @@ def resolve_collection_argument(
     found module.
 
     If the path doesn't exist, raise UsageError.
+    If the path is a directory and selection parts are present, raise UsageError.
     """
     strpath, *parts = str(arg).split("::")
     if as_pypath:
@@ -821,4 +822,11 @@ def resolve_collection_argument(
             else "file or directory not found: {arg}"
         )
         raise UsageError(msg.format(arg=arg))
+    if parts and fspath.is_dir():
+        msg = (
+            "package argument cannot contain :: selection parts: {arg}"
+            if as_pypath
+            else "directory argument cannot contain :: selection parts: {arg}"
+        )
+        raise UsageError(msg.format(arg=arg))
     return py.path.local(str(fspath)), parts

testing/test_main.py

@@ -136,23 +136,21 @@ class TestResolveCollectionArgument:
             ["foo", "bar", ""],
         )
 
-    def test_dir(self, root):
+    def test_dir(self, root: py.path.local) -> None:
         """Directory and parts."""
         assert resolve_collection_argument(root, "src/pkg") == (root / "src/pkg", [])
-        assert resolve_collection_argument(root, "src/pkg::") == (
-            root / "src/pkg",
-            [""],
-        )
-        assert resolve_collection_argument(root, "src/pkg::foo::bar") == (
-            root / "src/pkg",
-            ["foo", "bar"],
-        )
-        assert resolve_collection_argument(root, "src/pkg::foo::bar::") == (
-            root / "src/pkg",
-            ["foo", "bar", ""],
-        )
 
-    def test_pypath(self, root):
+        with pytest.raises(
+            UsageError, match=r"directory argument cannot contain :: selection parts"
+        ):
+            resolve_collection_argument(root, "src/pkg::")
+
+        with pytest.raises(
+            UsageError, match=r"directory argument cannot contain :: selection parts"
+        ):
+            resolve_collection_argument(root, "src/pkg::foo::bar")
+
+    def test_pypath(self, root: py.path.local) -> None:
         """Dotted name and parts."""
         assert resolve_collection_argument(root, "pkg.test", as_pypath=True) == (
             root / "src/pkg/test.py",
@@ -165,10 +163,11 @@ class TestResolveCollectionArgument:
             root / "src/pkg",
             [],
         )
-        assert resolve_collection_argument(root, "pkg::foo::bar", as_pypath=True) == (
-            root / "src/pkg",
-            ["foo", "bar"],
-        )
+
+        with pytest.raises(
+            UsageError, match=r"package argument cannot contain :: selection parts"
+        ):
+            resolve_collection_argument(root, "pkg::foo::bar", as_pypath=True)
 
     def test_does_not_exist(self, root):
         """Given a file/module that does not exist raises UsageError."""