Introduce path Clean/Join helper functions

[wxiaoguang]

Since #23493 has conflicts with latest commits, this PR is my proposal for fixing #23371

Details are in the comments

And refactor the modules/options module, to make it always use "filepath" to access local files.

Benefits:

• No need to do util.CleanPath(strings.ReplaceAll(p, "\\", "/"))), "/") any more (not only one before)
• The function behaviors are clearly defined

[techknowlogick]

CI Fail is related with:

-- FAIL: TestBuildLocalPath (0.00s)
    --- FAIL: TestBuildLocalPath/0/a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a14 (0.00s)
panic: FilePathJoinAbs: result is not absolute, do not guess a relative path based on current working directory [recovered]
	panic: FilePathJoinAbs: result is not absolute, do not guess a relative path based on current working directory
...

[wxiaoguang]

CI Fail is related with: TestBuildLocalPath

Fixed by cb9c518 , I think it's better to use absolute paths internally as much as possible, it avoids unclear behaviors.

[codecov-commenter]

Codecov Report

Merging #23495 (e94155b) into main (f521e88) will decrease coverage by 0.02%.
The diff coverage is 37.89%.

❗ Current head e94155b differs from pull request most recent head 4191802. Consider uploading reports for the commit 4191802 to get more accurate results

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@@            Coverage Diff             @@
##             main   #23495      +/-   ##
==========================================
- Coverage   47.14%   47.12%   -0.02%     
==========================================
  Files        1149     1154       +5     
  Lines      151446   152353     +907     
==========================================
+ Hits        71397    71795     +398     
- Misses      71611    72079     +468     
- Partials     8438     8479      +41     

Impacted Files	Coverage Δ
cmd/dump.go	0.67% <0.00%> (ø)
cmd/web.go	0.00% <0.00%> (ø)
models/actions/run.go	1.64% <0.00%> (-0.08%)	⬇️
models/actions/runner.go	1.44% <ø> (ø)
models/packages/package.go	45.45% <0.00%> (-1.13%)	⬇️
models/user/search.go	77.50% <0.00%> (-6.29%)	⬇️
modules/actions/github.go	0.00% <0.00%> (ø)
modules/actions/workflows.go	0.00% <0.00%> (ø)
modules/context/context.go	64.54% <0.00%> (-3.53%)	⬇️
modules/doctor/storage.go	31.93% <0.00%> (ø)
... and 33 more

... and 37 files with indirect coverage changes

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[wxiaoguang]

Made some new commits, major changes:

• Refactor the modules/options module, to make it always use "filepath" to access local files
• Extract some common code to general functions, like listLocalDirIfExist, readLocalFile, then the dynamic.go and static.go could be simplified a lot.
• The StaticRootPath was never well-defined, it could be a relative path, so introduce mustLocalPathAbs to tolerate such unstable behavior: make it absolute to current working directory if it was relative, just the same as before.

[wxiaoguang]

To address the concern about "misunderstand or misuse the SafePathXxx", in e94155b :

• Use PathJoinRel instead of SafePathRel, because it's only generally safe across elements, but it doesn't prevent the "/../" inside a single path element, it's caller's duty to define every single path's rule. Add related comments.
• Add tests suggested by Introduce path Clean/Join helper functions #23495 (comment)

[jolheiser]

🎺 🤖

[wxiaoguang]

Backport: Introduce path Clean/Join helper functions, partially backport&refactor #23607