refactor: divide pacquet-fs into multiple files

[KSXGitHub]

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

[codecov]

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 ⚠️
crates/fs/src/ensure_file.rs	88.46%	3 Missing ⚠️

Additional details and impacted files

@@           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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[github-actions]

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]

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

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