This commit fixes panic occurring if $DENO_DIR is set to a relative
path, eg. "DENO_DIR=denodir deno run main.ts".
Before creating DenoDir instance given path is checked and if necessary
resolved against current working directory.
Additional sanity checks were put in place to ensure all caches
receive absolute path for the location.
* 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
