prefixUrl is ignored causing URL building to fail

[Mnkras]

In fetch.ts there is this code:
const url = new URL(typeof input === 'string' ? input : input.url); that assumes that the input will always be a full URL or object with a full URL.
That is an incorrect assumption and breaks got clients using prefixUrl since input is just a path (/something)

It seems all this logic is to merge query params which references this got bug: sindresorhus/got#1188
Since it is now resolved, do we still need this code?

Can we remove this extra parsing?

[alexghr]

Based on #370, restoring the deleted test and running it against the new code (that doesn't separate search params) still fails so I think we still need the code. I have made suggestion to make both prefixUrl and searchParams merging work together.