Summary:
Disabling `atime` on Linux is quite common. (I don't have data to back this up,
but with my sample size of n=1, 100% fall into this bucket.) In that case,
the plugins will be cached indefinitely.
Using `ctime` on the directory doesn't really mean anything because it is
only affected by changes *to* the directory, not the files inside.
So, let's do this right and use the most recent change to any of the files
*inside* the directory instead.
Reviewed By: danielbuechele
Differential Revision: D9479491
fbshipit-source-id: 6945d7bf87defa67679cacdaf0a978d8ff1770c3