refactor: divide pacquet-fs into multiple files

[KSXGitHub]

Another PR requires this refactor. I make this a separate PR for easy diff.

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (c62a631) 92.65% compared to head (41f6b05) 92.65%.

Files	Patch %	Lines
crates/fs/src/file_mode.rs	33.33%	6 Missing :warning:
crates/fs/src/ensure_file.rs	88.46%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #214 +/- ## ======================================= Coverage 92.65% 92.65% ======================================= Files 57 59 +2 Lines 2847 2847 ======================================= Hits 2638 2638 Misses 209 209 ```

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Micro-Benchmark Results

Linux

group                          main                                   pr
-----                          ----                                   --
tarball/download_dependency    1.01      6.8±0.34ms   640.7 KB/sec    1.00      6.7±0.15ms   650.0 KB/sec

[github-actions[bot]]

Integrated-Benchmark Report (Linux)

Scenario: Frozen Lockfile

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
pacquet@HEAD	128.8 ± 12.2	112.7	149.8	1.04 ± 0.13
pacquet@main	124.1 ± 10.8	109.8	144.8	1.00

BENCHMARK_REPORT.json

```json { "results": [ { "command": "pacquet@HEAD", "mean": 0.1287552813657143, "stddev": 0.012223158123635705, "median": 0.12481295008, "user": 0.05480472000000001, "system": 0.20804341714285715, "min": 0.11265351808, "max": 0.14976112608, "times": [ 0.14845050208000002, 0.14341390908, 0.12753799808, 0.12661904208, 0.12299944908, 0.12929328708, 0.14976112608, 0.11619144708000001, 0.12004258908000001, 0.11761429508, 0.12299175608, 0.14199816208000002, 0.12300685808, 0.11265351808 ], "exit_codes": [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] }, { "command": "pacquet@main", "mean": 0.12411295676750002, "stddev": 0.010812024404156372, "median": 0.12408124708000001, "user": 0.05777034500000001, "system": 0.20566568500000001, "min": 0.10980995408000001, "max": 0.14480860308000001, "times": [ 0.12382968908, 0.12611078308, 0.11321339408, 0.13218096308000002, 0.13284036408, 0.12428732108000001, 0.13267985408, 0.12594394108, 0.10980995408000001, 0.11923158408000001, 0.14480860308000001, 0.11184873208, 0.11111872708000001, 0.12387517308000001, 0.11252745608, 0.14150076908 ], "exit_codes": [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ] } ] } ```