Remove circular dependency issue between btcec/v2 and main package

[guggero]

Fixes #1822.

After merging this PR we need to push two tags: chaincfg/chainhash/v1.0.0 and btcec/v2/v2.1.1.
And I think the two tags cannot be on the same commit (because go mod is just mean that way), so I'd suggest adding chaincfg/chainhash/v1.0.0 to 7cc824e and btcec/v2/v2.1.1 to 4ad74cd.

cc @Roasbeef.

[coveralls]

Pull Request Test Coverage Report for Build 1956823903

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 79.676%

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Totals
Change from base Build 1954395449:	0.0%
Covered Lines:	1231
Relevant Lines:	1545

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💛 - Coveralls

[guggero]

I added an updated version of #1780 as a commit to this PR as well (since the btcutil/go.sum needed to be updated).

EDIT: Removed that commit again since the author of the original PR already pushed a fix.

[Roasbeef]

I merged #1780, so I think this just needs a rebase now. I think we'll also need to tag new btcec (etc) versions after that prior (and this) merge, so we can bundle them after this lands.

[Roasbeef]

After merging this PR we need to push two tags: chaincfg/chainhash/v1.0.0 and btcec/v2/v2.1.1.
And I think the two tags cannot be on the same commit (because go mod is just mean that way

Weird, I think we've done this in the past? Though maybe there's some other sneaky interaction here.

[Roasbeef]

LGTM 🦟

Changes look good to me, if we really want to then we can eliminate the spew dep, but that only relies on the std-lib, so it doesn't bring any new deps into the fray.

[Roasbeef]

Needs aa rebase, then we can merge this and push out the new tags, then update the mod files in another PR.

[guggero]

Rebased!

[guggero]

Again, I think it is very important to push the tags for the correct commits! Otherwise we might point to a version that has a replace directive, creating more pain points.

Here are the updated commit hashes to tag:

• chaincfg/chainhash/v1.0.0: 7cc824e
• btcec/v2/v2.1.1 4ad74cd