Support multiple alias files

- To ease organisation and allow specialisation: aliases may be named with a suffix (such as ```alias.ec2```, ```alias.s3```)
- To accelerate collaboration (say, by allowing easy merging of pull requests)

Start by locating the alias-file loading entry point and any existing tests for alias discovery. Trace how one alias file is selected and determine how suffixed files could be supported without disrupting current behavior. Done means multiple named alias files can be organized and merged as described, with tests covering the new behavior.

Written by the indexing model from the issue text.