refactor: use utf8 strings as mem cache keys

[KSXGitHub]

Initially, the use of OsString was to allow non-UTF8 paths. However, the index file feature requires serde which in turns require UTF-8 to serialize. So the need for OsString ceases to be necessary, and removing it open new opportunities for some micro-optimizations.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6d8cec2) 92.68% compared to head (c2d89d3) 92.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #222 +/- ## ======================================= Coverage 92.68% 92.68% ======================================= Files 57 57 Lines 2871 2872 +1 ======================================= + Hits 2661 2662 +1 Misses 210 210 ```

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[github-actions[bot]]

Micro-Benchmark Results

Linux

group                          main                                   pr
-----                          ----                                   --
tarball/download_dependency    1.01      6.7±0.28ms   650.0 KB/sec    1.00      6.6±0.21ms   654.7 KB/sec

[github-actions[bot]]

Integrated-Benchmark Report (Linux)

Scenario: Frozen Lockfile

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
pacquet@HEAD	127.2 ± 9.8	114.5	144.3	1.03 ± 0.11
pacquet@main	123.2 ± 9.8	107.5	137.7	1.00

BENCHMARK_REPORT.json

```json { "results": [ { "command": "pacquet@HEAD", "mean": 0.1271716063466667, "stddev": 0.009812358487650672, "median": 0.12619836648, "user": 0.060102726666666675, "system": 0.2001150733333333, "min": 0.11450732448000002, "max": 0.14428669348, "times": [ 0.14428669348, 0.11990778348000002, 0.12619836648, 0.13605476948, 0.12718388648, 0.11450732448000002, 0.12066137548000001, 0.13758830448, 0.12542734648, 0.12743158148, 0.13692406248, 0.11953791848000002, 0.14075418748000001, 0.11592374748000002, 0.11518674748000002 ], "exit_codes": [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] }, { "command": "pacquet@main", "mean": 0.12316214754666668, "stddev": 0.009758218348705645, "median": 0.12175780848000001, "user": 0.05872752666666667, "system": 0.20377233999999997, "min": 0.10750951548000001, "max": 0.13768289148, "times": [ 0.13554187048, 0.11585921048000002, 0.13768289148, 0.11726989848000001, 0.11095596748000001, 0.11996457548000002, 0.12027272148000001, 0.13042775948, 0.11030591948000001, 0.13455231048, 0.12728008348, 0.12497080248000002, 0.12175780848000001, 0.13308087848, 0.10750951548000001 ], "exit_codes": [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] } ] } ```

[zkochan]

That's fine. Neither pnpm nor the other node.js based alternatives support non-utf8 chars.