AddOptionalFieldsToSchema code imported from Google's terraform provider.

FindExact finds the first element in 'slice' matching the condition defined by 'finder'.

FixDatasourceSchemaFlags is a convenience func that toggles the Computed, Optional + Required flags on a schema element.

SchemaFromResourceSchema is a recursive func that converts an existing Resource schema to a Datasource schema.

SingularDataSourceFindError returns a standard error message for a singular data source's non-nil resource find error.