Pull Request for Issue #35015 .

Summary of Changes

Added str_replace() to change \ to /.
Looks like only the str_replace() at line 189 maybe essential but included one at line 175 for consistency and clarity.

Testing Instructions

Use a MS Windows hosted test site (may not be a problem on Linux sites).
Install this library:
lib_bftest1.zip

Actual result BEFORE applying this Pull Request

The library installed correctly AND appears in the Extensions:Discover listing.
Can then be installed again from there - get 2 records in the extensions table!

Need to uninstall both before testing the patch.

Expected result AFTER applying this Pull Request

The library installed correctly and does not appear in the Extensions:Discover listing.

Documentation Changes Required

docs.joomla.org/Manifest_files
Has been updated with J4.0 library related information since originally written ...

Current documentation on manifest files for libraries is rather limited (no longer true!).
https://docs.joomla.org/Manifest_files

More information on this can be found elsewhere, such as ...
https://www.joomlashack.com/blog/how-tos/development/how-to-package-joomla-libraries/ (see comments at end)
https://joomla.stackexchange.com/questions/13681/manifest-file-format-for-installing-a-library-package

This pull request and previous pull request #34910 appear to complete an 'almost working' undocumented feature, namely ...

What is not immediately obvious is that you can include a slash (/) in the folder name, so you can put all your company libraries into JPATH_SITE/libraries/mycompany using this:

<libraryname>mycompany/mydosomething</libraryname>
<libraryname>mycompany/mydosomethingelse</libraryname>