Add Referenced By and Dependent Code analysis for assemblies

[pieroviano]

Shows which loaded assemblies reference a given assembly, and which of their members actually use it:

• Each assembly node in the tree gets a Referenced By folder listing the loaded assemblies that reference it (version mismatches are marked).
• Analyze now works on an assembly node. It adds the whole module to the Analyzer pane with two analyzers: Referenced By and Dependent Code. Dependent Code covers signatures, base types, constraints, method bodies and custom attributes, including attributes on parameters and types named in attribute arguments.
• Assembly and assembly-reference nodes get Referenced By / Dependent Code context-menu entries. They are disabled when the reference does not resolve to a loaded assembly.
• Assembly identity matching uses name, culture and public key token, but ignores version.

Try it: right-click an assembly > Dependent Code, or expand its Referenced By folder.

Split out of #4199 (one feature per PR, as requested there).

Screenshots