* remove fetch_source_file_and_maybe_compile_async and
replace it with State.fetch_compiled_module
* remove SourceFile.js_source()
* introduce CompiledModule which is basically the same as
deno::SourceInfo and represents arbitrary file that has been
compiled to JS module
* introduce //cli/compilers module containing all compilers
* introduce JsCompiler which is a no-op compiler
- output is the same as input, no compilation takes place
- it is used for MediaType::JavaScript and MediaType::Unknown
* introduce JsonCompiler that wraps JSON in default export
* support JS-to-JS compilation using checkJs
* merge SourceFileFetcher trait and FileFetcher struct
* move logic related to source file fetching to //cli/file_fetcher.rs
* use Result when creating new ThreadSafeState
