Directive class in generated API docs imported from async-directive instead of directive

[AndrewJakubowicz]

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At https://lit.dev/docs/api/custom-directives/#Directive the import of:

import { Directive } from 'lit/async-directive.js'; could more tersely written as import { Directive } from 'lit/directive.js';

Although both are correct. Maybe both should be shown?

[augustjk]

All the lit/directive.js exports on that page are showing lit/async-directive.js since I assume typedoc found it from that file first. Ideally, I think we could show both import sources, with the exception of Directive which should only show lit/directive.js. No idea if that's even feasible though. I think preferring lit/directive.js as their original source works fine if that's easier to do.