refactor: cleanup compiler pipeline (#2686)

* 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

cli/deno_dir.rs

@@ -17,8 +17,6 @@ pub struct DenoDir {
 }
 
 impl DenoDir {
-  // Must be called before using any function from this module.
-  // https://github.com/denoland/deno/blob/golang/deno_dir.go#L99-L111
   pub fn new(custom_root: Option<PathBuf>) -> std::io::Result<Self> {
     // Only setup once.
     let home_dir = dirs::home_dir().expect("Could not get home directory.");