Replace a republished shipping option, and reset the manifest per cart

[kha333n]

Fixes #2691.

The same code is on 2.x at Manifests/ShippingManifest.php, so this wants a
forward port.

A modifier that re-prices an option has its answer discarded, and in any process
handling more than one cart the next cart is offered the last one's options.

The fix

Two changes, one for each half of it.

addOption() replaces an option whose identifier is already present instead
of returning without doing anything. That is what makes a re-priced option stick.

getOptions() starts with a clean collection when the cart being resolved
changes. Resetting unconditionally would have been simpler, but much of the
suite publishes straight onto the manifest rather than from inside a modifier —
tests/Pest.php, tests/stripe/Utils/CartBuilder.php and seven others — and
that usage is legitimate. Resetting on a change of cart leaves it working while
still giving each cart a manifest of its own. There is a test pinning that.

In shipped code the only caller of addOption() is table-rate-shipping's
modifier, so nothing outside the tests depends on the old behaviour.

Tests

tests/core/Unit/Base/ShippingManifestTest.php:

• can replace an option that is published again — the same identifier at
  500 then 2500. Fails on 1.x with
  Failed asserting that 500 matches expected 2500.
• can not offer one cart the options priced for another — a second cart in
  the same process. Fails on 1.x with
  Failed asserting that actual size 1 matches expected size 0.
• can keep options published for the cart being resolved — an option added
  directly, then two resolves of that same cart. Passes before and after, and is
  the guard that this does not break the way the suite sets shipping up.