Revert packaging='pom' behavior change introduced in c5823ad. Fixes #810

.  Ref #636.

ivy/src/main/scala/sbt/Ivy.scala

@@ -245,14 +245,17 @@ private object IvySbt
 			// Technically, this should be applied to module configurations.
 			// That would require custom subclasses of all resolver types in ConvertResolver (a delegation approach does not work).
 			// It would be better to get proper support into Ivy.
+			// A workaround is to configure the ModuleConfiguration resolver to be a ChainResolver.
 			//
 			// This method is only used by the pom parsing code in Ivy to find artifacts it doesn't know about.
 			// In particular, a) it looks up source and javadoc classifiers b) it looks up a main artifact for packaging="pom"
 			// sbt now provides the update-classifiers or requires explicitly specifying classifiers explicitly
-			// Providing a main artifact for packaging="pom" does not seem to be correct and the lookup can be expensive, so
- 			// sbt now requires this artifact to be explicitly declared.
-			override def locate(artifact: IArtifact) = null
-//				if(hasImplicitClassifier(artifact)) null else super.locate(artifact)
+			// Providing a main artifact for packaging="pom" does not seem to be correct and the lookup can be expensive.
+			// 
+			// Ideally this could just skip the lookup, but unfortunately several artifacts in practice do not follow the
+			// correct behavior for packaging="pom" and so it is only skipped for source/javadoc classifiers.
+			override def locate(artifact: IArtifact) = if(hasImplicitClassifier(artifact)) null else super.locate(artifact)
+
 			override def getDependency(dd: DependencyDescriptor, data: ResolveData) =
 			{
 				if(data.getOptions.getLog != LogOptions.LOG_QUIET)

src/sphinx/Community/ChangeSummary_0.13.0.rst

@@ -13,7 +13,6 @@ Features, fixes, changes with compatibility implications (incomplete, please hel
 - Support for plugin configuration in ``project/plugins/`` has been removed.  It was deprecated since 0.11.2.
 - Dropped support for tab completing the right side of a setting for the ``set`` command.  The new task macros make this tab completion obsolete.
 - The convention for keys is now camelCase only.  Details below.
-- sbt no longer looks for main artifacts for poms with ``packaging="pom"``.  For details, see the :ref:`relevant Library Management section <packaging-pom>` and gh-636.
 - Fixed the default classifier for tests to be ``tests`` for proper Maven compatibility.
 - The global settings and plugins directories are now versioned.  Global settings go in ``~/.sbt/0.13/`` and global plugins in ``~/.sbt/0.13/plugins/`` by default.  Explicit overrides, such as via the ``sbt.global.base`` system property, are still respected.  (gh-735)
 - sbt no longer canonicalizes files passed to scalac.  (gh-723)

src/sphinx/Detailed-Topics/Library-Management.rst

@@ -513,23 +513,6 @@ This is confirmed by the output of ``show update``:
 
 **Note:** this is an Ivy-only feature and will not be included in a published pom.xml.
 
-
-.. _packaging-pom:
-
-packaging="pom"
-~~~~~~~~~~~~~~~
-
-A pom.xml that has `packaging="pom"` is not supposed to have artifacts.
-However, some published poms have an associated main artifact, so Ivy checks if one exists.
-This check can be time consuming to the point of taking most of the time for `update` on larger projects.
-Therefore, sbt disables this check and requires you to explicitly request the main jar.
-
-For example,
-
-::
-
-    libraryDependencies += "org.apache.velocity" % "velocity" % "1.5" jar()
-
 Publishing
 ~~~~~~~~~~