fix(vendor): handle relative imports when mapped local folder name differs from remote's

[dsherret]

I was using deno vendor this weekend and found this bug.

// https://localhost/npm:test/file.js
export function test() {}

// https://localhost/npm:other/file.js
import "../npm:test/file.js";

In this case, the npm:x folders will be mapped to npm_x. So when ../npm:test/file.js?test is resolved it can't find the folder because it's looking for ../npm_test. See the tests for more details.

Maybe this is an import map bug though and shouldn't be fixed in deno vendor? Also, perhaps there is a better way than this fix?

[bartlomieju]

LGTM!