Remove `DomainAssembly` concept

[elinor-fung]

DomainAssembly represented an assembly loaded (or being loaded) into a particular app domain - that is, there was a unique DomainAssembly per assembly per app domain. In our one app domain world, DomainAssembly versus Assembly is a confusing abstraction. We should move things off of DomainAssembly and eventually remove it.

Some things it currently handles:

• Exposed managed object for Assembly and Module - should be able to move to Assembly/Module
• Associated with AssemblyObject - can probably become Assembly
• FileLoadLevel, incremental loading based on level, and associated tracking/checks - maybe this - just load tracking - is what it becomes (renamed) or maybe collapse into Assembly
• Determining DebuggerAssemblyControlFlags for the assembly/module
• Used as main representation of a module in internal debug interfaces
• Tracking of collectible assemblies in the same ALC

[dotnet-policy-service]

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