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.

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
+}