add fallback for Go 1.7

x/mod/modfile is available from Go 1.8

gocover.go

@@ -16,7 +16,6 @@ import (
 	"path/filepath"
 	"strings"
 
-	"golang.org/x/mod/modfile"
 	"golang.org/x/tools/cover"
 )
 
@@ -113,15 +112,13 @@ func mergeTwoProfBlock(left, right []cover.ProfileBlock) []cover.ProfileBlock {
 
 // toSF converts profiles to sourcefiles for coveralls.
 func toSF(profs []*cover.Profile) ([]*SourceFile, error) {
+	// find root package to reduce build.Import calls when importing files from relative root
+	// https://github.com/mattn/goveralls/pull/195
 	rootDirectory, err := os.Getwd()
 	if err != nil {
 		return nil, fmt.Errorf("get working dir: %v", err)
 	}
-	modPath := filepath.Join(rootDirectory, "go.mod")
-	rootPackage := ""
-	if content, err := ioutil.ReadFile(modPath); err == nil {
-		rootPackage = modfile.ModulePath(content)
-	}
+	rootPackage := findRootPackage(rootDirectory)
 
 	var rv []*SourceFile
 	for _, prof := range profs {

gocover_ge1.8.go

@@ -0,0 +1,19 @@
+// +build go1.8
+
+package main
+
+import (
+	"io/ioutil"
+	"path/filepath"
+
+	"golang.org/x/mod/modfile"
+)
+
+func findRootPackage(rootDirectory string) string {
+	modPath := filepath.Join(rootDirectory, "go.mod")
+	content, err := ioutil.ReadFile(modPath)
+	if err != nil {
+		return ""
+	}
+	return modfile.ModulePath(content)
+}

gocover_lt1.8.go

@@ -0,0 +1,14 @@
+// +build !go1.8
+
+package main
+
+import (
+	"io/ioutil"
+	"path/filepath"
+
+	"golang.org/x/mod/modfile"
+)
+
+func findRootPackage(rootDirectory string) string {
+	return ""
+}