EndpointSliceCache was using the name "endpointInfo" to refer to two
different data types (most egregiously in addEndpoints(), which had a
variable named `endpoint` of type `*endpointInfo` and a variable named
`endpointInfo` of type `Endpoint`).
Continue using "endpointInfo" in places that refer to proxy.Endpoint /
BaseEndpointInfo, since that's consistent with other code, but rename
the local "cache of the Endpoints field of an EndpointSlice" type from
"endpointInfo" to "endpointData". Likewise, rename endpointSliceInfo
to endpointSliceData, for consistency.
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