SharedPartBuilder partId validation regex and error message are inconsistent

https://github.com/dart-lang/source_gen/blob/master/source_gen/lib/src/builder.dart

If `partId` provided to the SharedPartBuilder doesn't meet regex validation, an error is thrown:
```dart
if (!_partIdRegExp.hasMatch(partId)) {
throw ArgumentError.value(
partId,
'partId',
'`partId` can only contain letters, numbers, `_` and `.`. '
'It cannot start or end with `.`.',
);
}
```

It states `partId` can contain a period, however, here's the regex:
```dart
const partIdRegExpLiteral = r'[A-Za-z_\d-]+';

final _partIdRegExp = RegExp('^$partIdRegExpLiteral\$');
```

As you can see, no period is allowed.

Either the error message or regex should be updated to be consistent.

Open source_gen/lib/src/builder.dart and inspect SharedPartBuilder's partId validation and its ArgumentError message. Make the regex and message describe the same allowed characters, then run the relevant source_gen tests to confirm the validation and error wording are consistent.

Written by the indexing model from the issue text.