golang/tools: reduce jitter of packages.Load
In CL 614975, I have migrated deprecated loader from ssa package. However, miller has reported the tests became extremely slows. In an operation-system with a slow file system, such as plan9, the test slowed by more than 50x.
Oops, a un-intentional change:( I have submitted a CL to load 300+ packages by packages.Load once instead of loading them many. Alan thought it's a bravo change:)
Moreover, when I involved to investigate why the packages.Load slows so much, I have learned a lot. This blog records how I investigate them.
