Fix absolute label use in main attribute of py_test (#582)

Replaces #463 from @keith. We should get this in as part of our next
break.

In rules_python you can specify a file from another package like:

```bzl
main = "//path/to:file.py",
```

This didn't work in rules_py since it was taking the main as a string to
manipulate. This appears closer to me to the upstream logic that was
mirrored, so maybe this was changed for another reason

---

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes/no
- Breaking change (forces users to change their own code or config):
yes/no
- Suggested release notes appear below: yes/no

### Test plan

- [x] New test cases added

Co-authored-by: Keith Smiley <keithbsmiley@gmail.com>

Author: arrdem

examples/pytest/BUILD.bazel

@@ -38,3 +38,21 @@ py_test(
     shard_count = 2,
     deps = ["@pypi//pytest"],
 )
+
+py_test(
+    name = "absolute_main_test",
+    srcs = [
+        "__init__.py",
+        "main_test.py",
+    ],
+    main = "//examples/pytest:main_test.py",
+)
+
+py_test(
+    name = "main_with_colon_test",
+    srcs = [
+        "__init__.py",
+        "main_test.py",
+    ],
+    main = ":main_test.py",
+)

examples/pytest/__init__.py

@@ -0,0 +1 @@
+print("loaded as main!")

examples/pytest/main_test.py

@@ -25,8 +25,7 @@ def _determine_main(ctx):
         Artifact; the main file. If one can't be found, an error is raised.
     """
     if ctx.attr.main:
-        # Deviation from rules_python: allow a leading colon, e.g. `main = ":my_target"`
-        proposed_main = ctx.attr.main.removeprefix(":")
+        proposed_main = ctx.attr.main.label.name
         if not proposed_main.endswith(".py"):
             fail("main {} must end in '.py'".format(proposed_main))
     else:
@@ -87,7 +86,7 @@ determine_main = rule(
     implementation = _determine_main_impl,
     attrs = {
         "target_name": attr.string(mandatory = True, doc = "The name of the py_binary or py_test we are finding a main for"),
-        "main": attr.string(doc = "Hint the user supplied as the main"),
+        "main": attr.label(doc = "Hint the user supplied as the main", allow_single_file = True),
         "srcs": attr.label_list(allow_files = True),
     },
 )