The ModuleResolutionHost implementation inside ReflectorHost currently
relies on reading the snapshot to determine if a file exists, and use
the snapshot to retrieve the file content.
It is more straightforward and efficient to use the already existing
method fileExists() instead.
At runtime, the TypeScript LanguageServiceHost is really a Project, so
both fileExists() and readFile() methods are defined.
As a micro-optimization, skip fs lookup for tsx files.
PR Close#32642
bbb2798d41