Auto merge of #1812 - alexcrichton:moar-filtering, r=huonw

Auto merge of #1812 - alexcrichton:moar-filtering, r=huonw

Development dependencies are traversed during the resolution process, causing
them to be returned as the list of dependencies for a package in terms of
resolution. The compilation phase would then filter these out depending on the
dependency's transitivity, but this was incorrectly accounted for when the
dependency showed up multiple times in a few lists.

This commit fixes this behavior by updating the filtering logic to ensure that
only activated optional dependencies (those whose feature names are listed) are
compiled.

Closes #1805