multi: avoid direct use of dcrec/secp256k1 library

Because the original dcrec secp256k1 library that is used for the
Schnorr signature primitives uses different hash algorithms than the
btcd secp256k1 library. Therefore pulling in the wrong library can lead
to weird and unexpected errors. We try to make it harder to make the
mistake by not using the library directly in lnd in the first place.
Note that it is still indirectly needed by the btcd secp256k1 library,
therefore the module dependency is still expected to be there, just
moved to the indirect section.

go.mod

@@ -16,7 +16,6 @@ require (
 	github.com/btcsuite/btcwallet/wtxmgr v1.5.0
 	github.com/coreos/go-systemd v0.0.0-20190719114852-fd7a80b32e1f
 	github.com/davecgh/go-spew v1.1.1
-	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1
 	github.com/dvyukov/go-fuzz v0.0.0-20210602112143-b1f3d6f4ef4e
 	github.com/go-errors/errors v1.0.1
 	github.com/golang/protobuf v1.5.2
@@ -76,6 +75,7 @@ require (
 	github.com/coreos/go-systemd/v22 v22.3.2 // indirect
 	github.com/cpuguy83/go-md2man/v2 v2.0.0 // indirect
 	github.com/decred/dcrd/crypto/blake256 v1.0.0 // indirect
+	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.0.1 // indirect
 	github.com/decred/dcrd/lru v1.0.0 // indirect
 	github.com/dsnet/compress v0.0.1 // indirect
 	github.com/dustin/go-humanize v1.0.0 // indirect