ItemPage.fromPage should strip #section

When ItemPage.fromPage is asked to create an ItemPage object from
a Page, if the Page object has a 'section', it will send the #section
urlencoded to Wikibase API to be resolved to a data item, and Wikibase
will return -1 (missing).

Bug: 72317
Change-Id: Ifd6976f4132f827ddb12670dfe391ff7e7661d53