Move shards package to root and rename to 'search' (#930)
In https://github.com/sourcegraph/zoekt/pull/901 I moved several packages to 'internal' to clean up the exported API. This PR moves the `shards` package back to root, since it contains important methods like `NewDirectorySearcher`. It also renames the `shards` package to `search` to clarify the usage.
Relates to https://github.com/sourcegraph/zoekt/pull/901#issuecomment-2703171432, https://github.com/sourcegraph/zoekt/discussions/927