Attaching sources should use a project relative path, when possible

[fbricon]

when a source file is attached, an absolute path is added to the .classpath file. If the source file belongs to the project, then a relative path should be used instead, to keep .classpath as portable as possible.

See eclipse.jdt.ls/org.eclipse.jdt.ls.tests/projects/eclipse/source-attachment/.classpath after I attached the sources:

<?xml version="1.0" encoding="UTF-8"?>
<classpath>
	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8"/>
	<classpathentry kind="lib" path="foo.jar" sourcepath="/Users/fbricon/Dev/projects/eclipse.jdt.ls/org.eclipse.jdt.ls.tests/projects/eclipse/source-attachment/foo-sources.jar"/>
	<classpathentry kind="src" path="src"/>
	<classpathentry kind="output" path="bin"/>
</classpath>