internal/report: make openSourceFile cognizant of Go modules

This change adds $GOPATH/pkg/mod as a possible base to search
from given that Go modules have been the norm since Go1.11 and
we are currently at Go1.16/1.17, hence support for Go modules.

Fixes google#611

internal/report/source.go

@@ -917,8 +917,14 @@ func openSourceFile(path, searchPath, trim string) (*os.File, error) {
 		f, err := os.Open(path)
 		return f, err
 	}
+	possibleBases := filepath.SplitList(searchPath)
+	if gopath := os.Getenv("GOPATH"); gopath != "" {
+		// We can also look through the $GOPATH/pkg/mod in case the file originates
+		// from Go modules. See https://github.com/google/pprof/issues/611.
+		possibleBases = append(possibleBases, filepath.Join(gopath, "pkg", "mod"))
+	}
 	// Scan each component of the path.
-	for _, dir := range filepath.SplitList(searchPath) {
+	for _, dir := range possibleBases {
 		// Search up for every parent of each possible path.
 		for {
 			filename := filepath.Join(dir, path)