Well of course you can't use Ed25519 in ECDSA... That's EdDSA using Curve25519.

Curve25519 itself is originally a Montgomery  curve which should work fine for ECDSA. When you talk about Ed25519, you're actually talking about the Twisted Edwards form of Curve25519 instead.

The Go code you link @alex is the underlying ECC math operations on the Montgomery form of 25519. As far as I can see Go uses it there for NaCl, but has only implemented the crypto_box portion (which does ECDH) but not the signature portion (which would be EdDSA).