buildModuleRecordsFromImportedBuiltins(importMap) {
    return keys(importMap).reduce((acc, specifier) => {
      if (this.isBuiltin(specifier)) {
        /** @type {SimpleLavamoatModuleRecordOptions} */
        const lmrOpts = {
          type: LMR_TYPE_BUILTIN,
          file: specifier,
          specifier,
          packageName: specifier,
        }
        if (!this.#lmrCache.has(lmrOpts)) {
          this.#lmrCache.add(lmrOpts)
        }
        acc.push(
          /** @type {LavamoatModuleRecord} */ (this.#lmrCache.get(lmrOpts))
        )
      }
      return acc
    }, /** @type {LavamoatModuleRecord[]} */ ([]))
  }
Given an import map, creates an array of {@link LavamoatModuleRecord}s for
each builtin found in there.

@param {LavamoatModuleRecord['importMap']} importMap
@returns {LavamoatModuleRecord[]} Zero or more LMRs