Port fix for #1544 from Dependency to ExtractUsedNames

The fix for sbt/sbt#1237 was unfortunately not completely correct,
and infinite loops could still occur during the extraction of used
names.

In sbt/sbt#1544, a fix that was robuster and easier to understand
was applied to `/compile/interface/src/main/scala/xsbt/Dependency.scala`
in a similar situation (cyclic chains of original trees in macro
expansions).

This commit ports this fix to `ExtractUsedNames.scala`.

Closes sbt/sbt#1640, sbt/sbt#1610.
Merge pull request #1642 from Duhemm/fix-1640

Port fix for #1544 from Dependency to ExtractUsedNames