Skip manifests already found when scanning search paths

[Wint3rNight]

Fixes #1874.

Duplicate elimination runs on the search path strings, so it does not catch the case where one path names a directory and another names a file inside that directory. The two strings differ, but the directory scan and the explicit file path resolve to the same manifest, so the driver is loaded twice and its physical devices are reported twice.

With VK_DRIVER_FILES set to a folder plus two manifests inside that folder, vkEnumeratePhysicalDevices reports 9 devices where 5 exist, matching the log in the issue:

Original order:
      [0] pd10   [1] pd11   [2] pd00   [3] pd01
      [4] pd10   [5] pd11   [6] pd00   [7] pd01
      [8] pd0

Both call sites in add_data_files resolve to a full path before calling add_if_manifest_file, so comparing the resolved name catches the repeat. The existing copy_str_to_string_list_if_unique helper already does exactly this.

This is only visible with Vulkan 1.1 or newer, because GPDP2 gates the Linux physical device sort. The existing env var tests run at 1.0, which is why CI never caught it.

Added EnvVarICDOverrideSetup.DirAndFileOverlapDoesNotDuplicate, which fails on main with 9 devices and passes with the fix. Full suite is 703/703.

Two notes:

• add_data_files also serves layer discovery, so layers get the same protection. No existing test changes behaviour.
• loader_get_fullpath does not canonicalise symlinks or .. segments, so two different spellings of the same file are still not detected. That is outside what this issue reports and I have left it alone.

The issue also suggests running these env var cases at both 1.0 and 1.1, since much loader logic depends on GPDP2. That is a broader testing change and I have kept it out of this PR, but happy to follow up if useful.

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