Allow optional source directory to be added into project classpath

[jdneo]

Expected Behavior

Allow to add optional source folder classpath entry even the folder does not exist on disk.

Current Behavior

folder that does not exist on disk will be ignored.

Context

Currently, in https://github.com/eclipse/buildship/blob/35d01caad3ab023fdd9fdf4a1b9e9c738bff7fd0/org.eclipse.buildship.core/src/main/java/org/eclipse/buildship/core/internal/workspace/SourceFolderUpdater.java#L138

The source folder entry will be ignored if it does not exist.

There is one exception when the source folder has the optional attribute:

<attribute name="optional" value="true"/>

In JDT, optional classpath entry is allowed to be not existed - if I understood the JDT code correctly:

https://github.com/eclipse-jdt/eclipse.jdt.core/blob/f71a39afb62a1436cd094f6274ad9a91b96484f7/org.eclipse.jdt.core/model/org/eclipse/jdt/internal/core/ClasspathEntry.java#L2219-L2224

Steps to Reproduce

Try to add a non-exist source folder to classpath, sth like:

EclipseModel model = project.getExtensions().findByType(EclipseModel)
model.classpath.file.whenMerged { Classpath cp ->
  SourceFolder entry = new SourceFolder("<a path that does not exist on disk>", null)
  entry.entryAttributes.put("optional", "true")
  entry.entryAttributes.put("ignore_optional_problems", "true")
  cp.entries.add(entry)
}

More information

Another example is m2e, for example, if we import the spring-petclinic as a Maven project, m2e will generate classpath entries like:

<classpathentry kind="src" path="target/generated-sources/annotations">
	<attributes>
		<attribute name="optional" value="true"/>
		<attribute name="maven.pomderived" value="true"/>
		<attribute name="ignore_optional_problems" value="true"/>
		<attribute name="m2e-apt" value="true"/>
	</attributes>
</classpathentry>

while that generated folder does not exist on disk until build.