fix: Handle redirects as prefixes

[gibson042]

This fixes two general issues:

• broken references to sub-paths of moved paths, such as https://agoric.com/documentation/zoe/guide/contracts/covered-call.html (as seen at https://github.com/Agoric/agoric-sdk/blob/8406d71b74ba4abbc3c71afb536b2270f35c7cbc/packages/zoe/src/contracts/coveredCall.js#L16)
• external redirects, such as https://docs.agoric.com/guides/js-programming/ses/lockdown.html

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