Unconsidered linked .classpath files and test source folders

[HeikoKlare]

When using linked resources in an Eclipse project, they are not always properly considered by a Tycho build. This can particularly lead to missing consideration and/or execution of tests:

• A linked .classpath file is not considered, such that a JUnit test container specified in that file is not used for resolving JUnit, leading to compile failures of test classes.
• A linked test source folder is not considered, such that tests defined in that source folder are not compiled and executed.

The issue has been detected in eclipse-platform/eclipse.platform.swt#1203

I have provided a reproducer in #3804.

[HeikoKlare]

The root cause of this issue seems to be that the .classpath file is assumed to exist as direct content of a project's basedir:

tycho/tycho-metadata-model/src/main/java/org/eclipse/tycho/model/classpath/ClasspathParser.java

Lines 40 to 41 in 8728806

	public static Collection<ProjectClasspathEntry> parse(File basedir) throws IOException {
	File file = new File(basedir, ".classpath");