go.mod: switch to Go modules (#20311)

* go.mod, vendor: switch to Go modules

* travis: explicitly enable go modules in Go 1.11 and 1.12

* accounts/abi/bind: switch binding test to go modules

* travis, build: aggregate and upload go mod dependencies for PPA

* go.mod: tidy up the modules to avoid xgo writes to go.sum

* build, internal/build: drop own file/folder copier

* travis: fake build ppa only for go module dependencies

* mobile: fix CopyFile switch to package cp

* build, travis: use ephemeral debsrc GOPATH to get mod deps