Fixes several issues with the `HttpClient` migration that showed up after I tried updating the Material docs site to the latest v18 release. Includes:
* The migration was assuming that all Angular decorators have at least one argument. This led to a null pointer error that broke the v18 update process when I was testing it.
* The migration incorrectly reimplemented the detection of classes with Angular decorators. This can cause code to be migrated incorrectly and doesn't handle cases like import aliases. I've switched it to use the existing tooling for detecting decorated classes.
* The migration was trying to migrate directives, even though they don't support the `imports` field.
* The migration was detecting `TestBed.configureTestingModule` calls using a raw string match which is very fragile and can be broken by the user's formatting.
* There were syntax errors in the unit tests.
* There were type checking errors in the unit tests, for example none of them were importing the Angular decorators that they were migrating.

There's more room for improvement, but this should resolve the most glaring issues without having to rewrite too much.

PR Close #55557