Fix forward-referenced contract dependencies

[Jim8y]

• fix the compiler dropping forward-referenced smart contracts by collecting
  all smart contract symbols before building the dependency graph
• rebuild the dependency list using symbol equality so members referencing
  contracts declared later are preserved and _nonDependencies cannot trim
  required types
• guard against duplicate and abstract dependency entries while keeping the
  rest of the compilation flow unchanged

Issue:
CompilationEngine only added a smart contract to classDependencies after it
had already been visited, so a field or property that pointed to a contract
declared later looked like a plain class reference. When _nonDependencies
ran, those untracked contracts were considered foreign and stripped from the
compilation context, leading to partial NEF output in multi-contract projects.

Fix Details:
The change executes two passes over the syntax trees:

1. Record every public, non-abstract type that derives from SmartContract.
2. For each of those contracts, walk its members and add any referenced smart
   contract symbol to the dependency list, deduplicating and skipping abstract
   types.

This ensures the subsequent topological sort sees the full dependency graph
regardless of declaration order, so every required contract is compiled and
emitted.

[shargon]

No need to check if it's abstract here?