feat: log auto-discovered config file and support configFile: false

[dsherret]

Closes #399

The deno.json has been auto-discovered since 0.42 (#462), but there was no indication of which config file a build ended up using. Now it's logged:

[dnt] Transforming...
[dnt] Auto-discovered config file: /home/david/dev/my_project/deno.json
[dnt] Building project...

Additionally, configFile accepts false to opt out of the discovery entirely:

await build({
  // ...etc...
  configFile: false,
});

Details:

• The discovery already searches upwards from the entry points rather than the cwd, so the reported path is the config file that was actually used.
• Nothing is logged when a config file is explicitly provided, when an import map is provided (it's used as the config file), or when nothing was found.
• configFile: false maps to ConfigDiscoveryOption::Disabled, which also stops a package.json and deno.lock from being discovered. That's documented on the option and in the readme.
• configFile is now run through valueToUrl like importMap already was, so a plain path works instead of erroring while deserializing the specifier.
• Also fixes set_config_file in the rust test builder, which was setting the import map instead of the config file. The one test relying on it is renamed to match what it covers and a sibling test was added for the import map path.