Move root-level index code to index package (#902)
In the repo root, we have a bunch of low level logic around index building and
searching. So we end up exposing internal logic through the main public `zoekt`
package, for example `zoekt.Merge(...)`.
This PR moves it into the `build` package, so all code related to index
building lives together. It then renames `build` to `index` to reflect the
broader focus on indexing and searching the index.