Use IAssemblySymbol for tag helper discovery

[captainsafia]

Instead of expecting a MetadataReference to resolve tag helpers from, we'll take an IAssemblySymbol because:

• It plays nicer with the type that Rosyln exposes for the current compilation
• Avoids us having to repeatedly convert a MetadataReference to an IAssemblySymbol in the tag helper providers

Also added a check for applicable tag helper providers to avoid looking for tag helpers in assemblies that are not the target assembly.