module_resolution=none or =explicit

Discussed in-person with @DanielRosenwasser

In google we have performance issues with tsc and with editor plugins. We have tracked this down to the node moduleResolutionKind looking in many locations on a network-mounted disk.

We don't actually want the node moduleResolution, we only want the baseUrl/paths/rootDirs feature known as "path mapping". Put another way, we explicitly list locations for all inputs, and we never want the compiler to stat a file outside of the explicit input locations.

For tsc, we provide a custom CompilerHost that avoids touching the disk, we implement fileExists and directoryExists purely by consulting our file manifest. However, we can't do this trick for editors, which are becoming increasingly unusable.

As an example, we provide interop with Closure Compiler code that declares goog.module, by naming these goog:some.module. This results in the language services looking for a file like goog:some.module.d.ts all over the network-mounted filesystem, even though a later sourceFile in the program has declare module 'goog:some.module'

cc @vikerman @rkirov

Start with the tsc module resolution behavior and the language services used by editor plugins. Review how CompilerHost fileExists and directoryExists are consulted during module lookup, then define what an explicit or disabled module resolution mode should do for baseUrl, paths, rootDirs, and declared modules. Done means lookups avoid locations outside the explicit inputs without breaking path mapping.