Complete the transition of `core/types` to upstream code.
Note that the previous commit results in _all_ upstream types / vars / consts being in the `imports.go` file. This results in a strict partition of upstream and local identifiers so it is safe to simply change import paths.
1. Globally change all imports from `subnet-evm/core/types` to `libevm/core/types`;
2. Rename all `subnet-evm/core/types` imports to `customtypes` for ease of review here;
3. Dot import `libevm/core/types` in tests only to avoid hundreds of noisy changes.
See original PR https://github.com/ava-labs/coreth/pull/875
