* merge SourceFileFetcher trait and FileFetcher struct

* move logic related to source file fetching to //cli/file_fetcher.rs

* use Result when creating new ThreadSafeState

Symmetric with `readAll()` and `readAllSync()`. Also used in `xeval`.
Also correct usage in `writeFile()`/`writeFileSync()`.

* 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

https://github.com/denoland/deno/pull/2073#discussion_r303401539

So that "cargo build" will build it when the gn frontend is eventually
removed.

This is in preperation for dynamic import (#1789), which is more easily
implemented when errors are dynamic.