Improve generation performance by calling packages.Load only once

[LandonTClipp]

I've discovered that we can likely get a huge performance benefit by calling packages.Load only once. Currently, it is called once per file, which means that the entire dep tree is being loaded multiple times, thus resulting in O(n^2) performance. We should instead gather the list of files to load, then make one call to packages.Load. This should improve performance.

https://github.com/vektra/mockery/blob/master/pkg/parse.go#L76

https://github.com/vektra/mockery/blob/master/pkg/parse.go#L100

[LandonTClipp]

This will probably be implemented as part of #526