…ies on change

* snowpack/src/commands/dev.ts (installDependencies): import it.
(onDepWatchEvent): Call it.

* snowpack/src/sources/local-install.ts (InstallRunResult): export it.

* snowpack/src/sources/local.ts (InstallRunResult): import it.
(installDependencies): export it and give it a type.

Otherwise it's really easy to blow up the FS watcher limit.

* snowpack/src/commands/dev.ts: (startServer::depWatcher): Also
ignore stuff in config.exclude.

This is done rather brutally by clearing all the cache, but could
probably be done more criteriously.

* snowpack/src/commands/dev.ts (startServer::handleRequest): Improve
debug log message.
(startServer::handleRequest): Clear knownETags.