Make file locations slash-separated portable

[jserv]

Kconfig locations reached users with whatever separator the host used, so the same tree produced 'drivers/foo/Kconfig' on Linux but 'drivers/foo\bar/Kconfig' on Windows. The mixed form comes from the parser itself: 'rsource' builds its glob pattern with join(dirname(self.filename), pattern), and ntpath.join() inserts a backslash while the pattern text read from the Kconfig file keeps its forward slashes.

Normalize the $srctree-relative filename in _enter_file() so that Kconfig.filename, Kconfig.kconfig_filenames and MenuNode.filename are slash-separated everywhere. Absolute paths stay untouched, as kconfig_filenames has always documented, because they are inherently platform-specific and are the ones likely to be compared against os.path.realpath() output.

Apply the same rule to the top-level filename in _init(). Without it that one entry keeps the caller's separators while every sourced file is normalized, which leaves kconfig_filenames internally inconsistent and defeats the recursive 'source' check in _enter_file(): it compares _include_path entries against rel_filename, so 'menuconfig sub\Kconfig' on a self-sourcing file recursed until the stack blew instead of raising KconfigError.

Document the guarantee on the two public docstrings describing these attributes, and add test_filenames_are_slash_separated. Its slash assertions are inert on POSIX, where os.sep is already '/', but they pin the contract for the Windows runner, and the absolute-path assertion exercises the isabs() branch everywhere. Hoist the shared tests/Klocation environment setup into a klocation_env fixture instead of copying it into the new test.

Replace the _ordered_unique() seen-set idiom with
list(dict.fromkeys(lst)). Dict insertion order is guaranteed on every supported version, and both call sites hold Symbol/Choice objects that use identity hashing, so ordering and membership are unchanged.

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Summary by cubic

Makes relative Kconfig file locations slash-separated on every platform, so the same tree yields drivers/foo/Kconfig on Windows instead of drivers/foo\bar/Kconfig.

• Normalizes filenames in _enter_file() and the top-level file in _init() via _norm_loc(), and keeps $srctree prefix comparisons slash-separated; absolute paths stay untouched.
• Accepts pathlib.Path in Kconfig() and now always reports locations as strings.
• Fixes a Windows stack overflow where a self-sourcing file recursed forever instead of raising KconfigError.
• Corrects the location docs and adds the slash-separated guarantee on kconfig_filenames, MenuNode.filename, and Kconfig.filename.
• Replaces _ordered_unique() with list(dict.fromkeys(lst)).
• Tests the Windows rule by injecting ntpath into _norm_loc(), and covers the search_paths branch and absolute top-level filenames.

^{Written for commit 807577e. Summary will update on new commits.}