Fail packaging if a "files" pattern does not include any file

`vsce package` should fail here:
```json
{
	"files": [
           "this-file-does-not-exists.txt"
        ]
}
```

It should also fail here:
```json
{
	"files": [
           "this-folder-is-empty/**/*"
        ]
}
```

This will prevent countless of bugs that are very easy to run into.
There should be a flag `--ignore-empty-files-patterns` to disable this check.