Linked .classpath files: linked classpath entries paths not resolve against project base directory

[HeikoKlare]

Since #3806, linked .classpath files are supported. When using such a linked .classpath file, the path entries within that file are resolved against the folder containing the .classpath file instead of the project base directory, into which the .classpath file is linked.

Example: Having a project called PROJECT with a .classpath file in a linkedresources subfolder of a project, Tycho will resolve paths of classpath entries such as src_test to PROJECT/linkedresources/src_test instead of PROJECT/src_test. In consequence, paths used in linked .classpath files are currently not resolved correctly by Tycho.

Unfortunately, I had provided an integration test for the support of linked .classpath files in which a linked source folder is placed in the same folder as the linked .classpath file (https://github.com/eclipse-tycho/tycho/tree/main/tycho-its/projects/compiler.junitcontainer/junit5-with-linked-resources/linkedresources), so that the source folder is found (by accident) even though it is not resolved correctly by Tycho.

This is a follow-up to #3803.

@laeubi I missed this bug when validating against the SWT use case (eclipse-platform/eclipse.platform.swt#1203), as I had a slightly different configuration in my local setup when testing the improved Tycho version. I will provide a PR that corrects the behavior.