This PR improves the error output on publish when the `name` filed is
missing:
```json
{
"exports": "./mod.ts",
"version": "0.0.1"
}
```
Before:
```sh
deno publish --dry-run
error: You did not specify an entrypoint in file:///Users/marvinh/dev/test/deno-pkg-timers/deno.json. Add `exports` mapping in the configuration file, eg:
{
"name": "@scope/name",
"version": "0.0.0",
"exports": "<path_to_entrypoint>"
}
```
After:
```sh
deno publish --dry-run
error: Missing 'name' field in 'file:///Users/marvinh/dev/test/deno-pkg-timers/deno.json'.
```
Fixes https://github.com/denoland/deno/issues/27116
