Remove branchMap in prepareNormalBuild

[jtibshirani]

This PR refactors the "build preparation" code to remove branchMap, in favor of storing everything in a single map. The main motivation is just to make the code clearer.

Specific changes:

• Store branches in the same map as blob locations, remove branchMap
• Introduce new struct BlobIndexInfo to hold the old location information, plus branches
• Rename BlobLocation -> BlobRepo

To check there's no regression, I aadded a new benchmark BenchmarkPrepareNormalBuild. It shows small decrease in allocations:

Baseline vs. PR

BenchmarkPrepareNormalBuild-10    	       1	3649848792 ns/op	1632152832 B/op	 4260303 allocs/op
BenchmarkPrepareNormalBuild-10    	       1	3888314833 ns/op	1561963752 B/op	 4249175 allocs/op