Fix #102: Better main class detection

Previously, the main class detection was handled by
https://github.com/sbt/zinc/blob/1.0/internal/zinc-apiinfo/src/main/scala/xsbt/api/Discovery.scala
which looks for a main method with the correct signature in the
extracted API. This is imperfect because it relies on ExtractAPI
dealiasing types (because Discovery will look for a main method with a
parameter type of `java.lang.String` and won't recognize
`scala.Predef.String`), dealiasing means that the extracted API looses
information and thus can lead to undercompilation.

This commit partially fixes this by adding a new callback to AnalysisCallback:
    void mainClass(File sourceFile, String className)
that is used to explicitly register main entry points. This way, tools
do not need to interpret the extracted API, this is much better since it
makes it easier for zinc to evolve the API representation.

This commit does not actually changes ExtractAPI to not dealias, this
can be done in a later PR.

Note that there is another usecase for xsbt.api.Discovery that this PR
does not replace: discovering tests. This is more complicated because
different test frameworks have different ways to discover tests. For
more information, grep for "Fingerprint" in https://github.com/sbt/sbt
and https://github.com/sbt/junit-interface

Author: smarter

src/main/java/xsbti/AnalysisCallback.java

@@ -110,6 +110,20 @@ void generatedNonLocalClass(File source,
      */
     void api(File sourceFile, xsbti.api.ClassLike classApi);
 
+    /**
+     * Register a class containing an entry point coming from a given source file.
+     *
+     * A class is an entry point if its bytecode contains a method with the
+     * following signature:
+     * <pre>
+     * public static void main(String[] args);
+     * </pre>
+     *
+     * @param sourceFile Source file where <code>className</code> is defined.
+     * @param className A class containing an entry point.
+     */
+    void mainClass(File sourceFile, String className);
+
     /**
      * Register the use of a <code>name</code> from a given source class name.
      *
@@ -158,4 +172,4 @@ void problem(String what,
      * phase defined by <code>xsbt-analyzer</code> should be added.
      */
     boolean enabled();
-}
+}

src/main/java/xsbti/compile/analysis/SourceInfo.java

@@ -30,5 +30,12 @@ public interface SourceInfo {
      * @return The compiler reported problems.
      */
     public Problem[] getUnreportedProblems();
+
+    /**
+     * Returns the main classes found in this compilation unit.
+     *
+     * @return The full name of the main classes, like "foo.bar.Main"
+     */
+    public String[] getMainClasses();
 }
 

src/test/scala/xsbti/TestCallback.scala

@@ -64,6 +64,8 @@ class TestCallback extends AnalysisCallback {
     ()
   }
 
+  def mainClass(source: File, className: String): Unit = ()
+
   override def enabled(): Boolean = true
 
   def problem(category: String,