coursier should try all resolvers for missing classifiers

This is kind of shitty, but I would like that all resolvers be tried if a classifier is missing.

E.g.

`"com.viewpagerindicator" % "library" % "2.4.1" artifacts(Artifact("library", "aar", "aar"))`

is not available at http://repo1.maven.org/maven2/com/viewpagerindicator/library/2.4.1/

however, it is resolved at http://dl.bintray.com/populov/maven/com/viewpagerindicator/library/2.4.1/

Ivy seems to do this automatically, and so does gradle.

Start by tracing how coursier handles a missing classifier across its configured resolvers, using the com.viewpagerindicator 2.4.1 example as the reproduction. Compare the current behavior with the stated Ivy and Gradle behavior. Done means resolution continues to later resolvers and finds the artifact at the Bintray repository when it is absent from Maven Central.

Written by the indexing model from the issue text.