gnu: Move the Go standard libraries to (gnu packages golang).
* gnu/packages/syncthing.scm (go-golang-org-x-crypto-bcrypt,
go-golang-org-x-crypto-blowfish, go-golang-org-x-crypto-pbkdf2,
go-golang-org-x-crypto-tea, go-golang-org-x-crypto-salsa20,
go-golang-org-x-crypto-cast5, go-golang-org-x-crypto-twofish,
go-golang-org-x-crypto-xtea, go-golang-org-x-net-ipv4, go-golang-org-x-net-bpf,
go-golang-org-x-net-context, go-golang-org-x-net-internal-iana,
go-golang-org-x-net-ipv6, go-golang-org-x-net-proxy, go-golang-org-x-sys-unix,
go-golang-org-x-text-transform, go-golang-org-x-text-unicode-norm,
go-golang-org-x-time-rate): Move these packages ...
* gnu/packages/golang.scm: ... to here.
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