Optimize InstallPath.String()

Every InstallPath will have String() called on it eventually, often
more than once if it is in a slice that is sorted.  Precompute the
full path so it can be returned from InstallPath.String() without
recomputing every time.

Test: paths_test.go
Change-Id: I1ed0a3801806854356865c0a5fc35d5cf6d349fe