A crate can assign an alternative name, different from the crate name,
when importing another crate. On the command line this looks like:
rustc ... --extern foo_crate=path/to/bar_crate.rlib
We need to support this so we can eventually upgrade to rand-0.7.x.
* 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
* merge SourceFileFetcher trait and FileFetcher struct
* move logic related to source file fetching to //cli/file_fetcher.rs
* use Result when creating new ThreadSafeState
* 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
