Auto merge of #5490 - alexcrichton:fix-thrash, r=matklad
authorbors <bors@rust-lang.org>
Sat, 5 May 2018 20:41:12 +0000 (20:41 +0000)
committerbors <bors@rust-lang.org>
Sat, 5 May 2018 20:41:12 +0000 (20:41 +0000)
Factor in `used_in_plugin` to target filenames

This prevents thrashing the cache of compiled libraries for when they're used in
a plugin or not.


Trivial merge