Note for reviewer: it may be easier to look through this commit-by-commit since I took the chance to reorganize our tests a bit so they're easier to navigate. Includes the following changes:

fix(compiler-cli): correctly detect deferred dependencies across scoped nodes

This is based on an internal issue report.

An earlier change introduced a diagnostic to report cases where a symbol is in the deferredImports array, but is used eagerly. The check worked by looking through the deferred blocks in a scope, resolving the scope for each and checking if the element is within the scope. The problem is that resolving the scope won't work across scoped node boundaries. For example, if there's a control flow statement around the block or within the block but around the deferred dependency, it won't be able to resolve the scope since it isn't a direct child, e.g.

@if (true) {
  @defer {
   <deferred-dep/>
  }
}

To fix this the case where the deferred block is inside a scoped node, I've changed the R3BoundTarget.deferBlocks to be a Map holding both the deferred block and its corresponding scope. Then to resolve the case where the dependency is within a scoped node inside the deferred block, I've added a depth-first traversal through the scopes within the deferred block.

refactor(compiler-cli): move defer block tests into separate file

Splits the tests for @defer blocks out into a separate file since the ngtsc_spec.ts is getting quite large.