Remove vendor folder and use cached go mod/sum

[YoussB]

@pivotal-izabela-gomes brought to my attention that there is a vendor folder while we are using go mod to get the dependencies.

This PR:

• removes the vendor folder
• adds the go.mod and go.sum files early on in the build process so their steps are cached and thus builds will be faster.
• also, uses a different directory for the build process, since $GOPATH can't have go module files.