Remove trait `DependencyContext` in favor of enum

Since `DependencyContext` is needed in the compiler interface
subproject, it has to be defined in this same subproject.

`DependencyContext` is needed in this subproject because the
`AnalysisCallback` interface uses it.

compile/inc/src/main/scala/sbt/inc/Analysis.scala

@@ -5,6 +5,7 @@ package sbt
 package inc
 
 import xsbti.api.Source
+import xsbti.DependencyContext._
 import java.io.File
 
 /**

compile/inc/src/main/scala/sbt/inc/Compile.scala

@@ -10,6 +10,7 @@ import xsbti.{ Position, Problem, Severity }
 import Logger.{ m2o, problem }
 import java.io.File
 import xsbti.api.Definition
+import xsbti.DependencyContext._
 
 object IncrementalCompile {
   def apply(sources: Set[File], entry: String => Option[File],

compile/inc/src/main/scala/sbt/inc/Dependency.scala

@@ -2,22 +2,7 @@ package sbt.inc
 
 import java.io.File
 import xsbti.api.Source
-
-/**
- * Represents contextual information about particular depedency edge. See comments in
- * subtypes for examples of particular contexts.
- */
-private[inc] sealed abstract class DependencyContext
-
-/**
- * Marks dependency edge introduced by referring to a class through inheritance as in
- *
- *   class A extends B
- *
- * Each dependency by inheritance introduces corresponding dependency by member reference.
- */
-private[inc] final case object DependencyByInheritance extends DependencyContext
-private[inc] final case object DependencyByMemberRef extends DependencyContext
+import xsbti.DependencyContext
 
 /**
  * Represents the kind of dependency that exists between `sourceFile` and either `targetFile`

compile/inc/src/main/scala/sbt/inc/Relations.scala

@@ -8,6 +8,8 @@ import java.io.File
 import Relations.Source
 import Relations.SourceDependencies
 import xsbti.api.{ Source => APISource }
+import xsbti.DependencyContext
+import xsbti.DependencyContext._
 
 /**
  * Provides mappings between source files, generated classes (products), and binaries.

compile/persist/src/main/scala/sbt/inc/AnalysisFormats.scala

@@ -7,6 +7,7 @@ package inc
 import xsbti.api.{ Source, Compilation }
 import xsbti.{ Position, Problem, Severity }
 import xsbti.compile.{ CompileOrder, Output => APIOutput, SingleOutput, MultipleOutput }
+import xsbti.DependencyContext._
 import MultipleOutput.OutputGroup
 import java.io.File
 import sbinary._

interface/src/main/java/xsbti/DependencyContext.java

@@ -0,0 +1,22 @@
+package xsbti;
+
+/**
+ * Enumeration of existing dependency contexts.
+ * Dependency contexts represent the various kind of dependencies that
+ * can exist between symbols.
+ */
+public enum DependencyContext {
+	/**
+	 * Represents a direct dependency between two symbols :
+	 * object Foo
+	 * object Bar { def foo = Foo }
+	 */
+	DependencyByMemberRef,
+
+	/**
+	 * Represents an inheritance dependency between two symbols :
+	 * class A
+	 * class B extends A
+	 */
+	DependencyByInheritance
+}