fix(cache-tags): add `/index` to implicit tags and adjust `revalidatePath()` (#84586)
This fixes an issue where the tag was `/` but the build output was
`/index` (and vice-versa) meaning you couldn't use `revalidatePath()` in
some cases because the tag didn't match.
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Co-authored-by: JJ Kasper <jj@jjsweb.site>