Fixes: #2549

Context

When the eleventy ssr plugin loads modules concurrently, it would previously fail with an error. Something like:

Error: request for '@lit/reactive-element' is not yet fulfilled
        at SourceTextModule.link (node:internal/vm/module:198:17)
        at ModuleLoader.importModule (/home/aomarks/work/wireit-lit/packages/labs/ssr/src/lib/module-loader.ts:107:7)
        at async Promise.all (index 1)

Investigating this issue I found:

• Specification of the module methods: https://tc39.es/ecma262/#table-abstract-methods-of-module-records
• Located the error message in the node codebase: https://github.com/nodejs/node/blob/eed33c9deaa336e76c5385a077490107c6110f08/src/module_wrap.cc#L532-L535

Solution is to closer match the spec. LoadRequestedModules should prepare module for linking by loading all dependencies.
Then when Link is called, LoadRequestedModules must have completed.

Theory

Before linking our code does:

async importModule(...){
    ...
    const result = await this._loadModule(specifier, referrerPath); // <--- `LoadRequestedModules`
    const module = result.module as vm.Module;
    if (module.status === 'unlinked') {
      await module.link(this._linker); // <--- `Link`
    }
   ...
}

However previously our _loadModule method did not load all dependencies, relying on module.link to load the dependencies. This seems to work when not loading multiple modules concurrently.

But, to allow multiple modules to load concurrently I've added some additional module loading logic to _loadModule so dependencies get loaded.

The result is that by the time module.link is reached, all modules should have been loaded.

Test plan

I added a unit test which throws Error: request for 'XXXX' is not yet fulfilled without the fix.

Risk

There is a risk that this adds a regression to the eleventy SSR plugin if something isn't quite right.