migrate syft/cataloger to syft/pkg/cataloger

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

syft/pkg/cataloger/golang/parse_go_mod.go

@@ -0,0 +1,63 @@
+package golang
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"sort"
+
+	"github.com/anchore/syft/syft/pkg"
+	"golang.org/x/mod/modfile"
+)
+
+// parseGoMod takes a go.mod and lists all packages discovered.
+func parseGoMod(path string, reader io.Reader) ([]pkg.Package, error) {
+	packages := make(map[string]pkg.Package)
+
+	contents, err := ioutil.ReadAll(reader)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read go module: %w", err)
+	}
+
+	file, err := modfile.Parse(path, contents, nil)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse go module: %w", err)
+	}
+
+	for _, m := range file.Require {
+		packages[m.Mod.Path] = pkg.Package{
+			Name:     m.Mod.Path,
+			Version:  m.Mod.Version,
+			Language: pkg.Go,
+			Type:     pkg.GoModulePkg,
+		}
+	}
+
+	// remove any old packages and replace with new ones...
+	for _, m := range file.Replace {
+		packages[m.New.Path] = pkg.Package{
+			Name:     m.New.Path,
+			Version:  m.New.Version,
+			Language: pkg.Go,
+			Type:     pkg.GoModulePkg,
+		}
+	}
+
+	// remove any packages from the exclude fields
+	for _, m := range file.Exclude {
+		delete(packages, m.Mod.Path)
+	}
+
+	pkgsSlice := make([]pkg.Package, len(packages))
+	idx := 0
+	for _, p := range packages {
+		pkgsSlice[idx] = p
+		idx++
+	}
+
+	sort.SliceStable(pkgsSlice, func(i, j int) bool {
+		return pkgsSlice[i].Name < pkgsSlice[j].Name
+	})
+
+	return pkgsSlice, nil
+}