Avoid calling packages.Load

[xtonyjiang]

(Paraphrased this issue from the now-deleted go-mockery repo)

I've noticed that calls to mockery have a decently high overhead due to calling packages.Load, which makes network calls. We can probably avoid the call to packages.Load by doing something like what golang/mock#420 does.

[LandonTClipp]

Definitely would like to see this implemented!

[LandonTClipp]

This has already been optimized by using the packages feature. packages.Load will only be called once, which confers the performance benefit we're looking for.