CommonJS model: forwardsOutside is skipped by the import reader and not memoized #35

Found in the PR #29 round-14 review. Two follow-ups on forwardsOutside, the cross-file check for "this file forwards its whole export into a file outside the model":

1. The incremental import reader skips it. module-importers.ts calls collectCommonJsExportEvidence directly. For a.js: module.exports = require('./esm'), resolution uses the pre-model path, but reader.commonJs(a.js) returns true and reExports returns a synthetic wildcard. Sync reopening therefore follows a model the resolver does not use. No edge difference has been found yet; it only widens reopening.
2. No memo. It reruns on every commonJsExportEvidence call (resolveModuleFile, readFile and a source compare per hop, up to 8 hops). resolveViaImport, matchReference and defaultModuleNamespace each call it again for the same file. Memoize per (context, file) and keep the record in a local in resolveViaImport.