feat(modules): Use modules

[ayang64]

Use go modules for dependency management.

• Add .mod and .sum files generated by go mod init

• Use a directory outside of $GOPATH for testing.

• Update docker files to use go 1.13.8 instead of 1.12 for most builds.

• Simplify go installation by piping from wget to tar. This avoids
  having to do any cleanup.

• Build and test in a directory outside of GOPATH since we're using
  modules now.

• Update Jenkinsfile to build using modules by building directly inside
  of $WORKSPACE instead of in $GOPATH.

• Use go mod download instead of dep ensure to collect dependencies

• Remove Gopkg related data files.

• Replace calls to "dep ensure" with "go mod download"

• Replace check for being outside of $GOPATH to a check for being INSIDE
  of $GOPATH which is wrong when we're using modules

• Map $DIR to /root/influxdb in docker run command

• Clone source into /root/influxdb which is outside of our $GOPATH.

Closes #

Describe your proposed changes here.

• Well-formatted commit messages
• Rebased/mergeable
• Tests pass

[dgnorton]

Did go mod pull dependency versions from the old Gopkg files? Did it encounter any errors while doing so? Just want to make sure we didn't inadvertently change the version of any dependencies.

[ayang64]

go mod copies the dependencies from the Gopkg.toml and Gopkg.lock files.

the only issues involve warnings about a couple of imports that don't follow the semantic versioning rules:

$ go mod init github.com/influxdata/influxdb
go: creating new go.mod: module github.com/influxdata/influxdb
go: copying requirements from Gopkg.lock
go: converting Gopkg.lock: stat github.com/googleapis/gax-go@v2.0.5: github.com/googleapis/gax-go@v2.0.5: invalid version: module contains a go.mod file, so major version must be compatible: should be v0 or v1, not v2
go: converting Gopkg.lock: stat github.com/google/go-github@dd29b543e14c33e6373773f2c5ea008b29aeac95: github.com/google/go-github@v0.0.0-20181009003523-dd29b543e14c: invalid version: go.mod has post-v0 module path "github.com/google/go-github/v18" at revision dd29b543e14c

but those seem to be problems with transitive dependencies -- not direct ones.

[dgnorton]

I notice in #16903 that @FiloSottile manually added entries for those two. Should this PR do the same?

[FiloSottile]

I added them manually and then ran go mod tidy which did away with them. Not sure if it impacted the graph though.

[dgnorton]

@FiloSottile thanks. Really appreciate the input on that.

[Elbehery]

Nice Work !!

Clone the branch locally && tested by the following :

• go mod vendor
• go mod tidy
• go fmt
• go build ./...
• go test ./...