Warn/error on unspecified external modules

[Rich-Harris]

At present, any imported modules that can't be found relative to base are assumed to be external. I kind of like that behaviour, but it is a footgun - it might be better if you had to explicitly say which modules are external:

esperanto.bundle({
  base: 'src',
  entry: 'main',
  external: [ 'fs', 'assert' ]
}).then(...);

Only real question is whether Esperanto should throw an error or just console.warn() if it encounters an external module that wasn't specified.

[Rich-Harris]

A related issue: esperantojs/gobble-esperanto-bundle#1. Sometimes, when moving files around, modules are assumed incorrectly to be external because the relative paths no longer resolve to the right place.

Some thoughts:

• Relative paths should never be treated as external. If they can't be found, it's an error. Simples
• When a module can't be found, the error message should include information about the importer as well as the importee, because otherwise it's hard to debug
• rollup does this properly, and insists on an array of pre-approved external module IDs. It's much nicer

[Rich-Harris]

Closing as wontfix, per #191. Rollup (with the default resolver) expects external modules to be specified explicitly, and will error if it can't find a module with a relative path