Remove builder interface

[bioball]

The goal of this PR is to separte the concerns of building a file from analysis and publishing diagnostics.

Each VirtualFile now returns the underlying PklModule as a completable future, which gets invalidated if its contents change.

• Add getModule(): CompletableFuture<PklModule> to VirtualFile
• Add DiagnosticsManager, which builds new diagnostics as the project's state changes
• Change Stdlib interface to provide VirtualFile instead of PklModule
• Don't file parser diagnostic events, because I anticipate that this logic will get replaced once we've swapped out the ANTLR parser with tree-sitter.

Miscellaneous: change various events to follow ADT pattern (use sealed interface)