Make search much faster

[39ali]

Objective

Project search sends one candidate per file to its worker pool, and each one
carried an owned Snapshot. Every field of Snapshot is cheap to clone except
always_included_entries, a Vec<Arc<RelPath>> holding one entry per
always-included file.

With a broad file_scan_inclusions such as **/*, that vector holds an entry per file in the project, so cloning it once per file made search O(files²).

Partially addresses #38799 (still needs to fix the huge memory usage and the occasional stutters) .

Solution

Share the snapshot behind an Arc instead, the consumer only reads id(), abs_path()
and root_name(), so nothing needs an owned copy.

Testing

Using the linux kernel repo, i searched for vmx_l1d_should_flush and netif_rx and its at least 60x faster on 5950x.
with file_scan_inclusions: ["**/*"]

Release Notes:

• Fixed project search being very slow on projects that set a broad file_scan_inclusions

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