Cleanup compiler pipeline

[bartlomieju]

I think we should return CompiledModule from fetch_source_file_and_maybe_compile_async:

struct CompiledModule {
   name: String,
   code: String,
}

It'd be made from:

• original source file for JS and Unknown media types
• original source file wrapped in export for JSON media type
• output code of compiled JS file for TypeScript media type

Originally posted by @bartlomieju in https://github.com/_render_node/MDExOlB1bGxSZXF1ZXN0Mjk2NzUxMTk3/timeline/more_items

Additionally:
#2636 (comment)

With refactored DenoDir flow for obtaining compiled modules looks much better but still it's not perfect.

Namely: for all modules TsCompiler.compile_async is called and it short-circuits for non-TS files. Additionally there's SourceFile.js_source() function that shouldn't exist on SourceFile.

Solution proposal:

1. change return type of TsCompiler.compile_async to CompiledModule returning only URL and compiled code
2. introduce JsCompiler and JsonCompiler structs. First one would be pass-through and second one would wrap JSON file source code into default export (see:

   deno/cli/deno_dir.rs

   Lines 43 to 62 in ac98bd8

   	impl SourceFile {
   	// TODO(bartlomieju): this method should be implemented on new `CompiledSourceFile`
   	// trait and should be handled by "compiler pipeline"
   	pub fn js_source(&self) -> String {
   	if self.media_type == msg::MediaType::TypeScript {
   	panic!("TypeScript module has no JS source, did you forget to run it through compiler?");
   	}
   	// TODO: this should be done by compiler and JS module should be returned
   	if self.media_type == msg::MediaType::Json {
   	return format!(
   	"export default {};",
   	str::from_utf8(&self.source_code).unwrap()
   	);
   	}
   	// it's either JS or Unknown media type
   	str::from_utf8(&self.source_code).unwrap().to_string()
   	}
   	}

   )
3. each compiler registers what file extensions/content type it can compile - NOTE: in case of JS files we might use either JsCompiler or TsCompiler depending on the settings (this will be referenced in separate issue)