fix the module path

The module was defined as github.com/ipld/go-codec-dagpb/dagpb, but the
repository is at github.com/ipld/go-codec-dagpb. This means that neither
can be resolved as a valid Go package import path; the former results in
a GitHub 404, and the latter results in a valid GitHub repository which
holds a different module.

Rod confirmed that the /dagpb suffix wasn't intentional, so just get rid
of it.

While at it, mod tidy.

go.mod

-module github.com/ipld/go-codec-dagpb/dagpb
+module github.com/ipld/go-codec-dagpb
 
 go 1.15
 

go.sum

@@ -14,7 +14,6 @@ github.com/minio/sha256-simd v0.1.1 h1:5QHSlgo3nt5yKOJrC7W8w7X+NFl8cMPZm96iu8kKU
 github.com/minio/sha256-simd v0.1.1/go.mod h1:B5e1o+1/KgNmWrSQK08Y6Z1Vb5pwIktudl0J58iy0KM=
 github.com/mr-tron/base58 v1.1.0/go.mod h1:xcD2VGqlgYjBdcBLw+TuYLr8afG+Hj8g2eTVqeSzSU8=
 github.com/mr-tron/base58 v1.1.2/go.mod h1:BinMc/sQntlIE1frQmRFPUoPA1Zkr8VRgBdjWI2mNwc=
-github.com/mr-tron/base58 v1.1.3 h1:v+sk57XuaCKGXpWtVBX8YJzO7hMGx4Aajh4TQbdEFdc=
 github.com/mr-tron/base58 v1.1.3/go.mod h1:BinMc/sQntlIE1frQmRFPUoPA1Zkr8VRgBdjWI2mNwc=
 github.com/mr-tron/base58 v1.2.0 h1:T/HDJBh4ZCPbU39/+c3rRvE0uKBQlU27+QI8LJ4t64o=
 github.com/mr-tron/base58 v1.2.0/go.mod h1:BinMc/sQntlIE1frQmRFPUoPA1Zkr8VRgBdjWI2mNwc=
@@ -26,15 +25,12 @@ github.com/multiformats/go-multibase v0.0.1/go.mod h1:bja2MqRZ3ggyXtZSEDKpl0uO/g
 github.com/multiformats/go-multibase v0.0.3 h1:l/B6bJDQjvQ5G52jw4QGSYeOTZoAwIO77RblWplfIqk=
 github.com/multiformats/go-multibase v0.0.3/go.mod h1:5+1R4eQrT3PkYZ24C3W2Ue2tPwIdYQD509ZjSb5y9Oc=
 github.com/multiformats/go-multihash v0.0.10/go.mod h1:YSLudS+Pi8NHE7o6tb3D8vrpKa63epEDmG8nTduyAew=
-github.com/multiformats/go-multihash v0.0.13 h1:06x+mk/zj1FoMsgNejLpy6QTvJqlSt/BhLEy87zidlc=
 github.com/multiformats/go-multihash v0.0.13/go.mod h1:VdAWLKTwram9oKAatUcLxBNUjdtcVwxObEQBtRfuyjc=
 github.com/multiformats/go-multihash v0.0.14 h1:QoBceQYQQtNUuf6s7wHxnE2c8bhbMqhfGzNI032se/I=
 github.com/multiformats/go-multihash v0.0.14/go.mod h1:VdAWLKTwram9oKAatUcLxBNUjdtcVwxObEQBtRfuyjc=
-github.com/multiformats/go-varint v0.0.5 h1:XVZwSo04Cs3j/jS0uAEPpT3JY6DzMcVLLoWOSnCxOjg=
 github.com/multiformats/go-varint v0.0.5/go.mod h1:3Ls8CIEsrijN6+B7PbrXRPxHRPuXSrVKRY101jdMZYE=
 github.com/multiformats/go-varint v0.0.6 h1:gk85QWKxh3TazbLxED/NlDVv8+q+ReFJk7Y2W/KhfNY=
 github.com/multiformats/go-varint v0.0.6/go.mod h1:3Ls8CIEsrijN6+B7PbrXRPxHRPuXSrVKRY101jdMZYE=
-github.com/polydawn/refmt v0.0.0-20190807091052-3d65705ee9f1 h1:CskT+S6Ay54OwxBGB0R3Rsx4Muto6UnEYTyKJbyRIAI=
 github.com/polydawn/refmt v0.0.0-20190807091052-3d65705ee9f1/go.mod h1:uIp+gprXxxrWSjjklXD+mN4wed/tMfjMMmN/9+JsA9o=
 github.com/polydawn/refmt v0.0.0-20201211092308-30ac6d18308e h1:ZOcivgkkFRnjfoTcGsDq3UQYiBmekwLA+qg0OjyB/ls=
 github.com/polydawn/refmt v0.0.0-20201211092308-30ac6d18308e/go.mod h1:uIp+gprXxxrWSjjklXD+mN4wed/tMfjMMmN/9+JsA9o=
@@ -48,7 +44,6 @@ github.com/warpfork/go-wish v0.0.0-20200122115046-b9ea61034e4a h1:G++j5e0OC488te
 github.com/warpfork/go-wish v0.0.0-20200122115046-b9ea61034e4a/go.mod h1:x6AKhvSSexNrVSrViXSHUEbICjmGXhtgABaHIySUSGw=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190611184440-5c40567a22f8/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
-golang.org/x/crypto v0.0.0-20200117160349-530e935923ad h1:Jh8cai0fqIK+f6nG0UgPW5wFk8wmiMhM3AyciDBdtQg=
 golang.org/x/crypto v0.0.0-20200117160349-530e935923ad/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=
 golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad h1:DN0cp81fZ3njFcrLCytUHRSUkqBjfTo4Tx9RJTWs0EY=
 golang.org/x/crypto v0.0.0-20201221181555-eec23a3978ad/go.mod h1:jdWPYTVW3xRLrWPugEBEK3UY2ZEsg3UU495nc5E+M+I=
@@ -57,7 +52,6 @@ golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3/go.mod h1:t9HGtf8HONx5eT2rtn
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20190412213103-97732733099d/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20191026070338-33540a1f6037/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200122134326-e047566fdf82 h1:ywK/j/KkyTHcdyYSZNXGjMwgmDSfjglYZ3vStQ/gSCU=
 golang.org/x/sys v0.0.0-20200122134326-e047566fdf82/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201223074533-0d417f636930 h1:vRgIt+nup/B/BwIS0g2oC0haq0iqbV3ZA+u6+0TlNCo=
 golang.org/x/sys v0.0.0-20201223074533-0d417f636930/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=