external option does not work with plain specifier

[eibens]

#102 added support for marking specifiers as external, but it currently only works for specifiers such as foo:bar but not for a plain specifier such as bar.

The test for this indicates the possibility of marking bar as external, but does not actually test for this. If the test is extended to consider the bar import, like I did here, the test emits the following error:

✘ [ERROR] specifier was a bare specifier, but was not remapped to anything by importMap. [plugin deno-resolver]

    testdata/externals.ts:3:18:
      3 │ export { c } from "bar";
        ╵                   ~~~~~