archive/zip: reduce CPU usage in index construction

archive/zip: reduce CPU usage in index construction

Constructing the zip index (which is done once when first opening
a file in an archive) can consume large amounts of CPU when
processing deeply-nested directory paths.

Switch to a less inefficient algorithm.

Thanks to Jakub Ciolek for reporting this issue.

	goos: darwin
	goarch: arm64
	pkg: archive/zip
	cpu: Apple M4 Pro
	                          │  /tmp/bench.0  │            /tmp/bench.1            │
	                          │     sec/op     │   sec/op     vs base               │
	ReaderOneDeepDir-14         25983.62m ± 2%   46.01m ± 2%  -99.82% (p=0.000 n=8)
	ReaderManyDeepDirs-14          16.221 ± 1%    2.763 ± 6%  -82.96% (p=0.000 n=8)
	ReaderManyShallowFiles-14      130.3m ± 1%   128.8m ± 2%   -1.20% (p=0.003 n=8)
	geomean                         3.801        253.9m       -93.32%

Fixes #77102
Fixes CVE-2025-61728

Change-Id: I2c9c864be01b2a2769eb67fbab1b250aeb8f6c42
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3060
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/736713
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>