* cmd/dlv: fall back to go env GOPATH when GOPATH envvar is unset.
Since Go 1.8, GOPATH environment variable has a default value if the
environment variable is unset so not setting GOPATH is perfectly ok.
Depend on `go env` command to query Go related environment variable.
* build: update references to moved packages
Sometimes windows will send us events about breakpoints we have
already removed from the code despite the fact that we go to great
lengths to avoid this already.
Change waitForDebugEvent to check that when we receive a breakpoint
event the corresponding memory actually contains an INT 3
instruction, if it doesn't ignore the event and restart the thread.
Allows quoted substrings in build-flags flag. This fixes a build
problem on windows where the default build flags must contain a space.
Fixes#634 and #638
