Auto merge of #1371 - alexcrichton:issue-1289, r=huonw

Before this commit the `dep_targets` function of `Context` didn't actually
return all dependencies in the sense that unit tests (and likely examples) would
not have the same package's library listed as a dependency. This commit
rectifies the situation by ensuring that the package's library is included
whenever necessary in the dependency list.

Closes #1289