/usr/share/gocode/src/golang.org/x/crypto/curve25519/ is in golang-golang-x-crypto-dev 1:0.0~git20170629.0.5ef0053-2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/gocode/src/golang.org/x/crypto/curve25519/const_amd64.h
/usr/share/gocode/src/golang.org/x/crypto/curve25519/const_amd64.s
/usr/share/gocode/src/golang.org/x/crypto/curve25519/cswap_amd64.s
/usr/share/gocode/src/golang.org/x/crypto/curve25519/curve25519.go
/usr/share/gocode/src/golang.org/x/crypto/curve25519/curve25519_test.go
/usr/share/gocode/src/golang.org/x/crypto/curve25519/doc.go
/usr/share/gocode/src/golang.org/x/crypto/curve25519/freeze_amd64.s
/usr/share/gocode/src/golang.org/x/crypto/curve25519/ladderstep_amd64.s
/usr/share/gocode/src/golang.org/x/crypto/curve25519/mont25519_amd64.go
/usr/share/gocode/src/golang.org/x/crypto/curve25519/mul_amd64.s
/usr/share/gocode/src/golang.org/x/crypto/curve25519/square_amd64.s