This commit significantly refactors the 'typecheck' package to introduce a
new abstraction, the `TemplateTypeChecker`. To achieve this:
* a 'typecheck:api' package is introduced, containing common interfaces that
consumers of the template type-checking infrastructure can depend on
without incurring a dependency on the template type-checking machinery as
a whole.
* interfaces for `TemplateTypeChecker` and `TypeCheckContext` are introduced
which contain the abstract operations supported by the implementation
classes `TemplateTypeCheckerImpl` and `TypeCheckContextImpl` respectively.
* the `TemplateTypeChecker` interface supports diagnostics on a whole
program basis to start with, but the implementation is purposefully
designed to support incremental diagnostics at a per-file or per-component
level.
* `TemplateTypeChecker` supports direct access to the type check block of a
component.
* the testing utility is refactored to be a lot more useful, and new tests
are added for the new abstraction.
PR Close#38105
16c7441c2f