Add support for Go modules based on dvyukov#195 (comment)

go-fuzz-build/main.go

@@ -59,7 +59,6 @@ func makeTags() string {
 // that clients can then modify and use for calls to go/packages.
 func basePackagesConfig() *packages.Config {
  cfg := new(packages.Config)
- cfg.Env = append(os.Environ(), "GO111MODULE=off")
  return cfg
 }
 
@@ -235,7 +234,7 @@ func (c *Context) loadPkg(pkg string) {
  // See https://golang.org/issue/30826 and https://golang.org/issue/30828.
  rescfg := basePackagesConfig()
  rescfg.Mode = packages.NeedName
- rescfg.BuildFlags = []string{"-tags", makeTags()}
+ rescfg.BuildFlags = []string{"-mod=vendor", "-tags", makeTags()}
  respkgs, err := packages.Load(rescfg, pkg)
  if err != nil {
  c.failf("could not resolve package %q: %v", pkg, err)
@@ -255,7 +254,7 @@ func (c *Context) loadPkg(pkg string) {
  // of invalid code than trying to compile instrumented code.
  cfg := basePackagesConfig()
  cfg.Mode = packages.LoadAllSyntax
- cfg.BuildFlags = []string{"-tags", makeTags()}
+ cfg.BuildFlags = []string{"-mod=vendor", "-tags", makeTags()}
  // use custom ParseFile in order to get comments
  cfg.ParseFile = func(fset *token.FileSet, filename string, src []byte) (*ast.File, error) {
  return parser.ParseFile(fset, filename, src, parser.ParseComments)