Remove `swift_module_alias`

[brentleyjones]

Was originally intended to be like alias and just used for migration, but is easily misused to merge/reexport targets, which in turns causes problems for indexing, automation, etc. that try to map symbols back to targets.

Since it generates a new files, if dependent on multiple things to merge, that ends up being a choke point that slows downs builds when someone really only needs a subset of the dependencies.

So just remove it so it isn't something folks will try to reach for with many down sides.

If someone really needs the rexport steps for a migration, they can manually make a targets/srcs to do it, and hopefully thus be aware of the costs so there will migrate off it as soon as possible.

PiperOrigin-RevId: 528836559 (cherry picked from commit 4f27bd0d4217b96838190dfe9e3b99b32e420274)