This PR renames the `suggest` package to `search` and alters the layout slightly.
In the new package, the higher-level abstractions are on the top level and the search algorithms have their own subdirectories.
In a future refactor, we can turn algorithms such as PBT into actual `SearchAlgorithm` classes and move them into the `search` package.
The main reason to keep algorithms and searchers in the same directory is to avoid user confusion - for a user, `Bayesopt` is as much a search algorithm as e.g. `PBT`, so it doesn't make sense to split them up.
