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/compilers/json.rs

@@ -0,0 +1,26 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use crate::compilers::CompiledModule;
+use crate::compilers::CompiledModuleFuture;
+use crate::file_fetcher::SourceFile;
+use crate::state::ThreadSafeState;
+use std::str;
+
+pub struct JsonCompiler {}
+
+impl JsonCompiler {
+  pub fn compile_async(
+    self: &Self,
+    _state: ThreadSafeState,
+    source_file: &SourceFile,
+  ) -> Box<CompiledModuleFuture> {
+    let module = CompiledModule {
+      code: format!(
+        "export default {};",
+        str::from_utf8(&source_file.source_code).unwrap()
+      ),
+      name: source_file.url.to_string(),
+    };
+
+    Box::new(futures::future::ok(module))
+  }
+}