internal/fuzz: centralize corpus entry addition

Adds an addCorpusEntry method to coordinator which manages checking for
duplicate entries, writing entries to the cache directory, and adding
entries to the corpus. Also moves readCache to be a method on the
coordinator.

Fixes #50606

Change-Id: Id6721384a2ad1cfb4c5471cf0cd0a7510d250a6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/360394


Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>