Process each compilation separately when searching for embedded contracts

[Saancreed]

When building project A that references project B, and project A targets net8.0 while B targets net6.0, attempting to load all the external references into a single MetadataLoadContext explodes horribly with exceptions like

The assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' has already loaded been loaded into this MetadataLoadContext.
At
   at System.Reflection.MetadataLoadContext.LoadFromStreamCore(Stream peStream)
   at System.Reflection.MetadataLoadContext.LoadFromAssemblyPath(String assemblyPath)
   at System.Reflection.PathAssemblyResolver.Resolve(MetadataLoadContext context, AssemblyName assemblyName)
   at System.Reflection.MetadataLoadContext.TryFindAssemblyByCallingResolveHandler(RoAssemblyName refName)
   at System.Reflection.MetadataLoadContext.ResolveToAssemblyOrExceptionAssembly(RoAssemblyName refName)
   at System.Reflection.MetadataLoadContext.TryResolveAssembly(RoAssemblyName refName, Exception& e)
   at System.Reflection.MetadataLoadContext.TryGetDefaultCoreAssembly(Exception& e)
   at System.Reflection.MetadataLoadContext.TryGetCoreAssembly(String coreAssemblyName, Exception& e)
   at System.Reflection.TypeLoading.CoreTypes..ctor(MetadataLoadContext loader, String coreAssemblyName)
   at System.Reflection.MetadataLoadContext..ctor(MetadataAssemblyResolver resolver, String coreAssemblyName)
   at LeanCode.ContractsGenerator.Compilation.ContractsCompiler.TryLoadEmbeddedContracts(IReadOnlyCollection`1 compilations) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Compilation/ContractsCompiler.cs:line 144
   at LeanCode.ContractsGenerator.Compilation.ContractsCompiler.CompileProjectsAsync(IEnumerable`1 projectPaths, ImmutableDictionary`2 properties) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Compilation/ContractsCompiler.cs:line 60
   at LeanCode.ContractsGenerator.Program.HandleProjectAsync(ProjectOptions p) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Program.cs:line 175
   at LeanCode.ContractsGenerator.Program.Main(String[] args) in /home/runner/work/contractsgenerator/contractsgenerator/src/LeanCode.ContractsGenerator/Program.cs:line 115

[Patonymous]

LGTM, and seems to be working