…fixes apache#11381) (apache#11394)

This commit fixes the regression where resources with a relative targetPath
were being copied to the project root instead of relative to the output
directory (target/classes or target/test-classes).

Changes:

1. DefaultSourceRoot.fromModel: Store targetPath as a relative path instead
   of resolving it against baseDir and outputDir. This ensures that
   SourceRoot.targetPath() returns a relative path as intended by the
   Maven 4 API javadoc.

2. ConnectedResource.computeRelativeTargetPath: Simplified to directly
   return the relative targetPath from SourceRoot, since it's now always
   stored as relative.

3. Updated tests to expect relative paths from SourceRoot.targetPath().

Maven 4 API Conformance:
- SourceRoot.targetPath() returns an Optional<Path> containing the explicit
  target path, which should be relative to the output directory (or absolute
  if explicitly specified as absolute).
- SourceRoot.targetPath(Project) resolves this relative path against the
  project's output directory to produce an absolute path.

Maven 3 Compatibility:
- Resource.getTargetPath() in Maven 3 was always relative to the output
  directory. This behavior is preserved by storing targetPath as relative
  in SourceRoot and converting it back to relative for the Resource API
  via ConnectedResource.

Example: With <targetPath>custom-dir</targetPath>:
- Maven 3: Resources copied to target/classes/custom-dir
- Maven 4 (before fix): Resources copied to project-root/custom-dir
- Maven 4 (after fix): Resources copied to target/classes/custom-dir

Fixes apache#11381

(cherry picked from commit 9b95526)