feat: Support DOCFX_SOURCE_REPOSITORY_URL

[filzrev]

This PR intended to fix issue #9743.

When running SamplesTest on a forked docfx repository.
A lot of diffs are raised regarding git-related path differences.

This PR rewrites these paths if DOCFX_SOURCE_REPOSITORY is set.

---

This PR rewrites these paths with the following conditions.

1. When EnvironmentContext.IsSnapshotTestContext=true is explicitly set.
   (Used for in-process tests)
2. When DOCFX_IS_SNAPSHOT_TEST_CONTEXT environment variable is set.
   (Used for tests that launch docfx process)

Note

DOCFX_IS_SNAPSHOT_TEST_CONTEXT value is cached by Lazy<bool>.
Because GitUtility methods are called so many times. And I want to reduce Environment.GetEnvironmentVariable
~~system API call overhead. ~~

InternalVisiblesTo attributes are added to Docfx.Plugins project.
To reference EnvironmentContext.IsSnapshotTestContext internal property.
~~

[codecov]

Codecov Report

Attention: Patch coverage is 64.00000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 79.01%. Comparing base (fe673ec) to head (446fcdf).
Report is 98 commits behind head on main.

Files	Patch %	Lines
src/Docfx.Common/Git/GitUtility.cs	64.00%	4 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9759      +/-   ##
==========================================
+ Coverage   74.31%   79.01%   +4.69%     
==========================================
  Files         536      536              
  Lines       23189    23221      +32     
  Branches     4056     4037      -19     
==========================================
+ Hits        17234    18348    +1114     
+ Misses       4853     3744    -1109     
- Partials     1102     1129      +27     

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[filzrev]

Can repo URL be configured from the DOCFX_SOURCE_REPO_URL environment similar to how DOCFX_SOURCE_BRANCH_NAME works (and how branch are unified for PR builds)?

Changed environment variable name to DOCFX_SOURCE_REPOSITORY and set dotnet/docfx for snapshot test.
(Remove _URL postfix because it need to support non-GitHub repository URLs )

Other changes

• Add GitUtilityWithSourceRepoUrlTest to test organization/repository names override.
• Remove Environment variable caching. Because performance difference seem relatively small.
  (It's called about 1000 times when generating metadata for samples/seed projects)

[yufeih]

Thank you @filzrev !