[TrimmableTypeMap] Add ComputeTypeMapDependencies pre-task for partial builds

[simonrozsival]

Part of #10958.

The proposed per-assembly MSBuild partial-build design needs a lightweight dependency pre-task that computes which assemblies can affect each typemap unit. This issue tracks implementing that pre-task independently, without changing typemap generation yet.

Goal

Add a ComputeTypeMapDependencies-style MSBuild task that reads assembly metadata and produces per-assembly dependency metadata suitable for later Inputs/Outputs wiring.

Proposed behavior

• Read each input assembly's AssemblyRef metadata using System.Reflection.Metadata/PEReader.
• Build the transitive assembly dependency graph for the resolved assembly set.
• Prune dependencies that cannot affect trimmable typemap generation, especially assemblies with no path to Java-peer-bearing assemblies such as Mono.Android or Java.Interop.
• Output items for each candidate typemap input assembly with metadata containing its relevant dependency closure.

Current related code

• src/Microsoft.Android.Sdk.TrimmableTypeMap/Scanner/AssemblyIndex.cs already reads metadata and type references, but it builds a richer scan index than this pre-task should need.
• src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.TypeMap.Trimmable.targets currently passes all assemblies directly to GenerateTrimmableTypeMap.

Acceptance criteria

• New task computes deterministic per-assembly dependency metadata from a set of resolved assemblies.
• Task is covered by unit tests using small fixture assemblies or existing test infrastructure.
• No behavior change to typemap generation yet; this is an enabling step for later partial-build wiring.