Extensions: optimization and code improvement follow-ups

[jcouv]

This issue is referenced in source.
It tracks locations that were identified as potentially needing optimization or some code improvement.

• Review MightContainExtensionMethods and TypesMightContainExtensionMethods. Can we rename?
• Can we optimize NamespaceSymbol.GetExtensionContainers by checking for MightContainExtensionMethods on assembly?
• Shouldn't we check !IsReferenceType || !IsStatic || IsGenericType inside MightContainExtensionMethods instead (in NamedTypeSymbol.GetExtensionContainers)? That may be trouble since MightContainExtensionMethods for PE symbols also considers struct containers

Relates to test plan #76130