Managed
In a managed package, changes to standard picklists can’t be packaged and deployed to subscriber orgs. To change values in subscriber orgs, you must manually add or modify the values in the target subscriber org.
More details can be found here in Components Available in Managed Packages
Unmanaged
Picklist values from standard fields can be retrieved from and deployed to orgs just like any other metadata. This metadata won't be in the package.xml
file itself, rather it will be in the Opportunity object's metadata file.
The package.xml
to retrieve or deploy this one field, will look like this:
<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
<types>
<members>Opportunity.Type</members>
<name>CustomField</name>
</types>
<version>57.0</version>
</Package>