Create new package when converting to blocks

Currently converting procs/functions to blocks for codegen creates the new block(s) in the existing package. It might be better if the new blocks were constructed in a new package. Few reasons:

1. The proc/function is no longer necessary for codegen so it just clutters the package with unnecessary garbage. Deleting the proc/function is a possibility, but even better would be to leave the input code to the block conversion unchanged.
2. Prevent name collisions between blocks and procs/functions. Right now they are allowed to have the same name .
3. Prevent channel name collision. We probably need a new kind of channel which encapsulates the data/ready/valid ports of the block. See https://github.com/google/xls/issues/554. Ideally this new channel type would have the same name as the channel it came from.