Bug

Fixes: NuGet/Home#14577

Description

The full implementation of aliasing allows multiple TargetFramework with the same effective tfm.
In order to achieve that, we will need to change up a bunch of the NuGet internal types to be identifiable by TargetAlias as well.
In this PR, I migrate our multiplexing to aliasing and attaching alias to 2 types that will inevitably need it, which is:

• LockFileTarget, the type that expresses the targets element in the assets file, which is currently pivoted off the effective TFM full name.
• IRestoreTargetGraph - the result type for a restore graph. Each tfm/rid combo currently gets its own IRestoreTargetGraph.
• Added FrameworkRuntimeDefinition (instead of reusing framework runtime pair, which itself contains the alias as well. Looking for feedback on the name here. I made the type internal, because we don't really need to expose, so we can easily iterate on the name.
• While both new and old resolver will label their RestoreTargetGraph results with TargetAlias, only the new one will eventually support full aliasing.
• "Supports" or compatibility profiles restore in the legacy resolver cannot really be supported correctly because some of the TFMs specified there are new and different from one originally specified, so we do a best effort and while it leads to an alias that doesn't exist, there is really no component that would actually use it. "Supports" is really just a restore thing.
• LockFile - I removed the unused methods. This just simplifies the amount of code we need to maintain.
• LockFileTarget - The restore code generates a LockFileTarget with an alias based on RestoreTargetGraph, but we we read it off disk, we need to patch it up, which is what Utf8JsonStreamLockFileConverter does.
  This is important because it allows everyone to depend on the TargetAlias regardless of what the lock file format itself is.
• PackageSpecExtensions - added ways to look up a TFI and PRMFI based on an alias instead of just a framework.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.