xgettext: Avoid collisions when splitting .pot files

[cip999]

Hey, I've been looking into #185 and its predecessor #67. While thinking of ways to implement the "reconstruction" of the catalog from smaller pieces, I noticed that the code doesn't handle well books where several part titles and/or chapters have the same name (or, more precisely, are equivalent up to slugging).

For example, consider the following SUMMARY.md:

# Summary

# Part Foo

[Chapter 1](src/chapter-1.md)

# Part Bar

[Chapter 2](src/chapter-2.md)

# Part Foo

[Chapter 3](src/chapter-3.md)

With a depth of 1, the preprocessor generates two POT files foo.pot and bar.pot (beside summary.pot), and the contents of the two Part Foo's (including Chapters 1 and 3) are merged. Since those are unrelated, if not for having the same title, the expected behavior would be to put them in separate files. Note that the titles don't have to be exactly equal, for example the same would happen with Part Foo 1 and Part Foo 2 (since numbers are discarded).

This PR should fix this issue. I hope doc comments are explicative enough, otherwise feel free to request clarifications. :)

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[cip999]

Ah, the PR is somwhat large so let me know if you want me to split it into 2-3 commits.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.34043% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.11%. Comparing base (0fdaff1) to head (aa7f82f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
i18n-helpers/src/xgettext.rs	97.34%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #243      +/-   ##
==========================================
+ Coverage   84.89%   85.11%   +0.22%     
==========================================
  Files          15       15              
  Lines        3369     3433      +64     
  Branches     3369     3433      +64     
==========================================
+ Hits         2860     2922      +62     
  Misses        422      422              
- Partials       87       89       +2     

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