Replace some casts from BuildRule to Buildable with rule.getBuildable().

Summary:
I think that we needed these casts at one time, but now they are no
longer appropriate. Especially since more of our implementations of
`BuildRule` do not actually implement `Buildable`.

Test Plan: Sandcastle builds.
diff --git a/src/com/facebook/buck/cli/TargetsCommand.java b/src/com/facebook/buck/cli/TargetsCommand.java
index 5f95da2..ed41f98 100644
--- a/src/com/facebook/buck/cli/TargetsCommand.java
+++ b/src/com/facebook/buck/cli/TargetsCommand.java
@@ -136,7 +136,7 @@
       String output = target.getKey();
       if (showOutput) {
         BuildRule buildRule = target.getValue();
-        String outputPath = ((Buildable)buildRule).getPathToOutputFile();
+        String outputPath = buildRule.getBuildable().getPathToOutputFile();
         if (outputPath != null) {
           output += " " + outputPath;
         }
diff --git a/src/com/facebook/buck/java/AnnotationProcessingParams.java b/src/com/facebook/buck/java/AnnotationProcessingParams.java
index 5110f85..3316c09 100644
--- a/src/com/facebook/buck/java/AnnotationProcessingParams.java
+++ b/src/com/facebook/buck/java/AnnotationProcessingParams.java
@@ -19,7 +19,6 @@
 import com.facebook.buck.rules.AnnotationProcessingData;
 import com.facebook.buck.rules.BuildRule;
 import com.facebook.buck.rules.BuildRuleResolver;
-import com.facebook.buck.rules.Buildable;
 import com.facebook.buck.rules.RuleKey;
 import com.facebook.buck.util.BuckConstant;
 import com.facebook.buck.util.HumanReadableException;
@@ -190,7 +189,7 @@
         // We're using raw strings here to avoid circular dependencies.
         // TODO(simons): don't use raw strings.
         if ("java_binary".equals(type) || "prebuilt_jar".equals(type)) {
-          String pathToOutput = ((Buildable)rule).getPathToOutputFile();
+          String pathToOutput = rule.getBuildable().getPathToOutputFile();
           if (pathToOutput != null) {
             searchPathElements.add(pathToOutput);
           }
diff --git a/src/com/facebook/buck/rules/BuildTargetSourcePath.java b/src/com/facebook/buck/rules/BuildTargetSourcePath.java
index 0e6d197..f90dd2f 100644
--- a/src/com/facebook/buck/rules/BuildTargetSourcePath.java
+++ b/src/com/facebook/buck/rules/BuildTargetSourcePath.java
@@ -41,7 +41,7 @@
       throw new HumanReadableException("Cannot resolve: %s", buildTarget);
     }
 
-    String path = ((Buildable)rule).getPathToOutputFile();
+    String path = rule.getBuildable().getPathToOutputFile();
     if (path == null) {
       throw new HumanReadableException("No known output for: %s", buildTarget);
     }