* 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.
