cmd/dlv: support multifile when dlv `debug`, `trace` and `test`
cmd/dlv: support multifile when dlv `debug`, `trace` and `test`

This changes make `dlv` support multifile specifically when dlv `debug`,
`trace` and `test`. Just like `dlv debug a.go b.go`. Corresponding to
`go build a.go b.go`.(a.go and b.go are  belong to `main` package).

Fix #984.

dr2chase/delve,dr2chase/delve,aarzilli/delve,go-delve/delve,go-delve/delve,derekparker/delve,alexbrainman/delve,alexbrainman/delve,aarzilli/delve,derekparker/delve,alexbrainman/delve,go-delve/delve,dr2chase/delve,derekparker/delve,go-delve/delve,go-delve/delve,aarzilli/delve,aarzilli/delve