This commit switches from dep to Go 1.12 modules for tracking Pulumi
dependencies. Rather than _building_ using Go modules, we instead use the `go
mod vendor` command to populate a vendor tree in the same way as `dep ensure`
was previously doing.
In order to prevent checksum mismatches, it was necessary to also update CI to
use Go 1.12 instead of 1.11 - which also necessitated fixing some linting errors
which appeared with the upgraded golangci-lint for 1.12.
