Linker: be better about loading adaptors from the filesystem

If you do openfn -a ../my-adaptor/, the linker will fail to load your adaptor module.

Technically we support passing a path as an adaptor. But we're not very good with paths, apparently.

Here's the problematic code in src/modules/linker.ts:

  // If the specifier is a path, just import it
  if (specifier.startsWith('/') && specifier.endsWith('.js')) {
    const importPath = `${prefix}${specifier}`;
    log.debug(`[linker] Loading module from path: ${importPath}`);
    return import(importPath);
  }

I mean, say no more.

We do have a well-tested function somewhere for detecting an absolute or relative path, so we should be able to handle this.

Start in src/modules/linker.ts and inspect the existing well-tested function for detecting absolute or relative paths. Exercise the openfn -a ../my-adaptor/ entry point and confirm that adaptor modules supplied by relative or absolute filesystem paths load successfully.