Use @ to specify alias, change ~ to :base

To prevent confusion regarding paths vs aliases, specify aliases using a
`@aliasName` format, though the alias definitions themselves should
still be of the format `{ aliasName: ':base/some/path' }`.  Also change
the "baseUrl" symbol from `~` to `:base` to prevent confusion on *nix
systems, where ~ is an alias to "user home directory."

Updated tests to match.