Add support for Go Modules

[kamilogorek]

I couldn't find any reliable way to extract current package name, as in here:

raven-go/stacktrace_test.go

Lines 109 to 135 in 238ebd8

	func derivePackage() (file, pack string) {
	// Get file name by seeking caller's file name.
	_, callerFile, _, ok := runtime.Caller(1)
	if !ok {
	return
	}
	// Trim file name
	file = callerFile
	for _, dir := range build.Default.SrcDirs() {
	dir := dir + string(filepath.Separator)
	if trimmed := strings.TrimPrefix(callerFile, dir); len(trimmed) < len(file) {
	file = trimmed
	}
	}
	// Now derive package name
	dir := filepath.Dir(callerFile)
	dirPkg, err := build.ImportDir(dir, build.AllowBinary)
	if err != nil {
	return
	}
	pack = dirPkg.ImportPath
	return
	}

due to Go Modules not using GOPATH anymore.
Need more investigation.

Some sources:
https://github.com/golang/go/wiki/Modules#migrating-to-modules
https://research.swtch.com/vgo-module

[tankbusta]

Is this issue why I'm seeing the raven package in the error title? (We're using modules)

[kamilogorek]

I don't think so, I was able to successfully read the module name using modules as well.

Do you have any repro of this behaviour?