[4.2] Fix library autoloading

[SharkyKZ]

Summary of Changes

Fixes #39347.

This fixes some issues in namespace mapper regarding library extension autoloading. First, this changes the manifest lookup from JPATH_LIBRARIES to JPATH_MANIFESTS directory where library manifests are actually stored. Secondly, this adds support for the possibly undocumented "vendor" library structure.

Testing Instructions

See #39342 and #39347.

Documentation Changes Required

Although this PR is a bug fix, library autoloading and "vendor" library structure features are probably not documented. So documentation should be added somewhere.

[sergeytolkachyov]

@SharkyKZ tested it successfully.

[joomdonation]

@sergeytolkachyov Could you please go to https://issues.joomla.org/tracker/joomla-cms/39348 , click on Test this button and report your test result there so that your test will be counted? We require two successful tests for each PR before it can be merged.

[sergeytolkachyov]

I have tested this item ✅ successfully on 326bfe2

Tested this successfully.

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[joomdonation]

Thanks @sergeytolkachyov

[richard67]

I've restored @sergeytolkachyov 's test result because the commits which have invalidated the test counter were just removing a redundant variable declaration and a clean branch update. The PR should still work the same as before.

But I think it is not easy to test without having an example library following that vendor structure.

[sergeytolkachyov]

I've restored @sergeytolkachyov 's test result because the commits which have invalidated the test counter were just removing a redundant variable declaration and a clean branch update. The PR should still work the same as before.

But I think it is not easy to test without having an example library following that vendor structure.

I'll test it again tomorrow.

[richard67]

I'll test it again tomorrow.

@sergeytolkachyov No need to do that. Your test is still valid, I restored the counter.

[sergeytolkachyov]

@richard67 what is currently missing for the merging of this pr?

[richard67]

@richard67 what is currently missing for the merging of this pr?

@sergeytolkachyov A 2nd human test. Your test still counts, but each PR needs 2 human tests before it is set to RTC (ready to commit). After that a maintainer will merge it whenever they find time.

[sergeytolkachyov]

I have tested this item ✅ successfully on 53f4b88

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[leha123ra]

I have tested this item ✅ successfully on 53f4b88

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[richard67]

RTC

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[roland-d]

Thank you