fix: propagate pytestmark from __init__.py to Package collector

[waterWang]

Fixes #14737

Problem

When a pytestmark is defined in a package's __init__.py, it is no longer propagated to tests within that package. This worked in earlier versions but was broken by a refactoring.

Root Cause

Package.setup() imports the __init__.py module via importtestmodule() but never reads the pytestmark attribute from it. The get_unpacked_marks() function (already imported) is used elsewhere (e.g., in PyobjMixin._filter_with_own_markers) to extract marks from module objects, but was never called for the __init__.py module in Package.setup().

Fix

Added get_unpacked_marks(init_mod) call in Package.setup() after importing the __init__.py module. The extracted marks are added to self.own_markers and self.keywords, which is consistent with how other collectors handle marks (see PyobjMixin at line 313 and FunctionDefinition.__init__ at line 1709).

Testing

• pytestmark = pytest.mark.skip(reason="package skip") in __init__.py now correctly skips tests in the package
• iter_markers() on package-level markers now includes marks from __init__.py
• All existing tests should pass as this is purely additive