Extract source code dependencies by tree walking.

Previously incremental compiler was extracting source code
dependencies by inspecting `CompilationUnit.depends` set. This set is
constructed by Scala compiler and it contains all symbols that given
compilation unit refers or even saw (in case of implicit search).
There are a few problems with this approach:

  * The contract for `CompilationUnit.depend` is not clearly defined
    in Scala compiler and there are no tests around it. Read: it's
    not an official, maintained API.
  * Improvements to incremental compiler require more context
    information about given dependency. For example, we want to
    distinguish between dependency on a class when you just select
    members from it or inherit from it. The other example is that
    we might want to know dependencies of a given class instead of
    the whole compilation unit to make the invalidation logic more
    precise.

That led to the idea of pushing dependency extracting logic to
incremental compiler side so it can evolve indepedently from Scala
compiler releases and can be refined as needed. We extract
dependencies of a compilation unit by walking a type-checked tree
and gathering symbols attached to them.

Specifically, the tree walk is implemented as a separate phase that
runs after pickler and extracts symbols from following tree nodes:

  * `Import` so we can track dependencies on unused imports
  * `Select` which is used for selecting all terms
  * `Ident` used for referring to local terms, package-local terms
            and top-level packages
  * `TypeTree` which is used for referring to all types

Note that we do not extract just a single symbol assigned to `TypeTree`
node because it might represent a complex type that mentions
several symbols. We collect all those symbols by traversing the type
with CollectTypeTraverser. The implementation of the traverser is inspired
by `CollectTypeCollector` from Scala 2.10. The
`source-dependencies/typeref-only` test covers a scenario where the
dependency is introduced through a TypeRef only.

interface/src/main/java/xsbti/AnalysisCallback.java

@@ -27,4 +27,16 @@ public interface AnalysisCallback
 	/** Provides problems discovered during compilation.  These may be reported (logged) or unreported.
 	* Unreported problems are usually unreported because reporting was not enabled via a command line switch. */
 	public void problem(String what, Position pos, String msg, Severity severity, boolean reported);
+	/**
+	 * Determines whether member reference and inheritance dependencies should be extracted in given compiler
+	 * run.
+	 *
+	 * As the signature suggests, this method's implementation is meant to be side-effect free. It's added
+	 * to AnalysisCallback because it indicates how other callback calls should be interpreted by both
+	 * implementation of AnalysisCallback and it's clients.
+	 *
+	 * NOTE: This method is an implementation detail and can be removed at any point without deprecation.
+	 *       Do not depend on it, please.
+	 */
+	public boolean memberRefAndInheritanceDeps();
 }

interface/src/test/scala/xsbti/TestCallback.scala

@@ -4,7 +4,7 @@ import java.io.File
 import scala.collection.mutable.ArrayBuffer
 import xsbti.api.SourceAPI
 
-class TestCallback extends AnalysisCallback
+class TestCallback(override val memberRefAndInheritanceDeps: Boolean = false) extends AnalysisCallback
 {
 	val sourceDependencies = new ArrayBuffer[(File, File, Boolean)]
 	val binaryDependencies = new ArrayBuffer[(File, String, File, Boolean)]