GRADLE-3022 Rename API from FileCopyDetails#getSourceRelativePath to FileCopyDetails#getRelativeSourcePath

+review REVIEW-5135
GRADLE-3022 Add interface override for FileCopyDetails#getName

The interface override allows to add some Javadoc that documents the modified behavior of FileCopyDetails#getName compared to the documented specification on FileTreeElement#getName.

+review REVIEW-5135
GRADLE-3022 Add API FileCopyDetails#getSourceName

+review REVIEW-5135
GRADLE-3022 Add integration tests for new APIs on FileCopyDetails

+review REVIEW-5135
GRADLE-3022 Improve name of test method

+review REVIEW-5135