I think that the second half of this sentence is not needed and arguably incorrect, because collections and spines are neither metadata nor resource information.
The package element is the root container of the Package Document and encapsulates metadata and resource information associated with an EPUB Package.
I'd argue that the spine is a form of metadata and collections provide more complete resource information than the manifest, but I think it's pointless to explain that the root element encapsulates everything inside the root element. We have a general description of what it encapsulates in the intro.
Proposed Solution:
Remove the prose from "and encapsulates" on.
I would keep it as it is since it does encapsulate both metadata and resource information.
If it encapsulates more than that, we should either enumerate all or remain silent.
We could change to:
"The package element is the root element of the Package Document. It defines various aspects of the EPUB Package, as described in the introduction."
This would keep it from being a generalized repetition.
"The package element is the root element of the Package Document. It defines various aspects of the EPUB Package, as described in the introduction."
Isn't it a problem to refer to the informative introduction in this normative definition?
Not for a description, no. If normative requirements slipped into the introduction we'd have a problem.
If "described" is off-putting, we could make the reference a parenthetical:
"The package element is the root element of the Package Document and defines various aspects of the EPUB Package (see the introduction for a general overview)."
Most helpful comment
We could change to:
"The package element is the root element of the Package Document. It defines various aspects of the EPUB Package, as described in the introduction."
This would keep it from being a generalized repetition.