Rename `blazecore` package to `blaze.core`

[Ben Plommer]

The current naming is confusing. Everything in blazecore is private[http4s] so this isn't binary-breaking. With the change, it can instead be private[blaze].

Alternatively it could just be renamed to blaze.

[Ross A. Baker]

That's a good idea. The blaze namespace didn't used to be "ours", while blaze was also positioned as a standalone product underneath http4s. Nobody has spoken for it as such in years, so we can claim org.http4s.blaze.core.

While it's still in a separate repo, we'd still be wise to avoid the root package org.http4s.blaze.