Mappings on single-jar legacy versions broken due to force-enabling split official namespaces

[Juuxel]

(Loom 1.11.7 on Gradle 9.0.0)

When targeting MC 0.30, Loom incorrectly uses the clientOfficial and serverOfficial namespaces of legacy merged versions despite 0.30 having no server jar. This means that mappings that use the correct official namespace no longer work, despite working in previous 1.x versions such as 1.5.

The clientOfficial and serverOfficial namespaces should not exist when there's only one environment available to begin with.

The specific issue that caused mappings to stop working was not adding those namespaces since namespace completion filled in the blanks. The bug was "fully" added in #1171 which stopped unwanted ns completion on split official ns versions to prevent mapping conflicts.

Reproducer (see the two branches on that repo): https://github.com/Juuxel/classic-mappings-reproducer

I can confirm from local testing that applying this patch to completely disable the merged namespace system single-jar envs with no intermediate mappings works.

[SpaceWalkerRS]

This should be fixed carefully, so as not to break single environment projects for pre-1.3 MC versions that do have both a client and a server jar. The check should probably be more specific and check that the version is between b1.0 and 1.3 instead of just check that it's older than 1.3.

[Juuxel]

My current WIP fix for this checks that both client and server are available in the version info's downloads, which should be enough.

[SpaceWalkerRS]

That would leave a handful of Beta/early release versions to be different than expected. For example, Beta 1.3_01 is client only, Beta 1.5_02 is server only, and so is release 1.0.1. Some versions have re-uploads of which only one side has been archived so far.

Having just the one system for a range of versions, even if it's technically unnecessary for the odd one in there, would be preferred. I am biased, though, since that's how we've been doing it over at Ornithe for a while now and I'd prefer not changing it or hack fixing it.