Update spec: Java library API references the APIs of other libraries
Update spec: Java library API references the APIs of other libraries

 - Postpone introduction of new `requires` syntax by introducing `api.dependencies`
 - Remove `exported(true)` from library selector
 - Merge and trim test cases

gradle/langos#23
+review REVIEW-5662

gstevey/gradle,robinverduijn/gradle,gstevey/gradle,blindpirate/gradle,blindpirate/gradle,lsmaira/gradle,gradle/gradle,lsmaira/gradle,robinverduijn/gradle,robinverduijn/gradle,robinverduijn/gradle,lsmaira/gradle,blindpirate/gradle,gradle/gradle,gstevey/gradle,gstevey/gradle,blindpirate/gradle,robinverduijn/gradle,blindpirate/gradle,lsmaira/gradle,robinverduijn/gradle,gstevey/gradle,gradle/gradle,gradle/gradle,robinverduijn/gradle,blindpirate/gradle,blindpirate/gradle,robinverduijn/gradle,gstevey/gradle,blindpirate/gradle,robinverduijn/gradle,gradle/gradle,gradle/gradle,gradle/gradle,lsmaira/gradle,lsmaira/gradle,lsmaira/gradle,robinverduijn/gradle,gradle/gradle,gradle/gradle,lsmaira/gradle,blindpirate/gradle,lsmaira/gradle,gstevey/gradle,gstevey/gradle,gradle/gradle,lsmaira/gradle,robinverduijn/gradle,blindpirate/gradle,gstevey/gradle