This removes dispatch_flatbuffers as it is now unused. There are still a
few places where msg_generated is used: ErrorKind and MediaType. These
will be dealt with later.

* Revert "port more ops to JSON (#2809)"

This reverts commit 137f3373.

* Revert "port ops to JSON: compiler, errors, fetch, files (#2804)"

This reverts commit 79f82cf1.

* Revert "Port rest of os ops to JSON (#2802)"

This reverts commit 5b2baa5c.

Fixes #1432

Just some clean up reorganization around flatbuffer/minimal dispatch
code. This is prep for adding a JSON dispatcher.

And throws without allow-env 

* 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

* rename `ModuleMetaData` to `SourceFile` and remove TS specific
  functionality

* add `TsCompiler` struct encapsulating processing of TypeScript files

* move `SourceMapGetter` trait implementation to `//cli/compiler.rs`

* add low-level `DiskCache` API for general purpose caches and use it in
  `DenoDir` and `TsCompiler` for filesystem access

* don't use hash-like filenames for compiled modules, instead use
  metadata file for storing compilation hash

* add `SourceFileCache` for in-process caching of loaded files for fast
  subsequent access

* define `SourceFileFetcher` trait encapsulating loading of local and
  remote files and implement it for `DenoDir`

* define `use_cache` and `no_fetch` flags on `DenoDir` instead of using
  in fetch methods

The rules are now as follows:

* In `import` statements, as mandated by the WHATWG specification,
  the import specifier is always treated as a URL.
  If it is a relative URL, it must start with either / or ./ or ../

* A script name passed to deno as a command line argument may be either
  an absolute URL or a local path.
  - If the name starts with a valid URI scheme followed by a colon, e.g.
    'http:', 'https:', 'file:', 'foo+bar:', it always interpreted as a
    URL (even if Deno doesn't support the indicated protocol).
  - Otherwise, the script name is interpreted as a local path. The local
    path may be relative, and operating system semantics determine how
    it is resolved. Prefixing a relative path with ./ is not required.

By making WASM compilation synchronous. We'll have to do more work to
make it properly async.

* Compiler no longer has its own Tokio runtime. Compiler handles one
  message and then exits.

* Uses the simpler ts.CompilerHost interface instead of
  ts.LanguageServiceHost.

* avoids recompiling the same module by introducing a hacky but simple
  `hashset<string>` that stores the module names that have been already
  compiled.

* Removes the CompilerConfig op.

* Removes a lot of the mocking stuff in compiler.ts like `this._ts`. It
  is not useful as we don't even have tests.

* Turns off checkJs because it causes fmt_test to die with OOM.