Auto-instrumentation when user's autoloader throws exception on not found

[labbati]

Description

This PR brings improvements to our auto-instrumentation and class loading sub-system.

Specifically, we could identify three major flaws:

• if a user's registered autoloader is not lenient, i.e. it is not designed for chaining because it throws an exception when it does not find the file in its own search paths, we caused an exception when DDTrace\Tracer class reference was tested for existence.
• Auto-instrumentation was 'indirectly' tested in tens of tests, but we lacked a specific suite that tested the different use cases for integration with users' autoloaders.
• If a user registered a dependency in the composer.json file we would have picked it in place of the version bundled with the extension, but this was the result of fortunate circumstances instead of an explicit workflow in the code.

This PR fixes all the above flaws, in addition to add a bunch of comments that explain why specific choices were made.

Readiness checklist

• Changelog entry added, if necessary
• Tests added for this feature/bug