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/mod.rs

@@ -0,0 +1,20 @@
+// Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
+use deno::ErrBox;
+use futures::Future;
+
+mod js;
+mod json;
+mod ts;
+
+pub use js::JsCompiler;
+pub use json::JsonCompiler;
+pub use ts::TsCompiler;
+
+#[derive(Debug, Clone)]
+pub struct CompiledModule {
+  pub code: String,
+  pub name: String,
+}
+
+pub type CompiledModuleFuture =
+  dyn Future<Item = CompiledModule, Error = ErrBox> + Send;