Treating relative paths as externals via resolveId does not work #763

Hi!

For a specific build I want to treat a (normal) (relative) import as an external resource. Given::

import {throttle} from './Lib/decorators'

throttle(/* function */)...

I want a build roughly as follows::

(function(Lib_decorators){ ... }(Lib.decorators))

I first tried resolveId with something maybe naive like if (importee.endsWith('Lib/decorators')) return false b/c you say that returning a "falsy value signals that importee should be treated as an external module and not included in the bundle" (https://github.com/rollup/rollup/wiki/Plugins).

But that doesn't work at all; it throws Could not resolve... and aborts. Reading the source code https://github.com/rollup/rollup/blob/master/src/Bundle.js#L228-L254 hints me that I have to use options.external which is otherwise not pluggable instead. But then rollup treats './Lib/decorators' and ../Lib/decorators (and so on) as different modules. Output::

(function (__Lib_decorators,___Lib_decorators) {
        'use strict';

}(__Lib_decorators,___Lib_decorators));

From my perspective the codepath I linked to is already too complex and just open for bugs.

The best workaround I found is letting resolveId return a name e.g. Lib.decorators, then treating this name as external via options.external, lastly rename the wrongly normalized name Lib_decorators back to Lib.decorators via options.globals::

const plugin = function() {
  return {
    resolveId(importee, importer) {
      if (importee.endsWith('Lib/decorators')) return 'Lib.decorators'
    }
  }
}

export default {
  format: 'iife',
  external: function(id) {
    if (id === 'Lib.decorators') return true
  },
  globals: function(id) {
    if (id === 'Lib.decorators') return 'Lib.decorators'
  },
  plugins: [ plugin() ]
}