feat: scan local go mod cache for licenses of golang packages (#1645)

Signed-off-by: Avi Deitcher <avi@deitcher.net>
Co-authored-by: Keith Zantow <kzantow@gmail.com>

internal/licenses/list.go

@@ -0,0 +1,53 @@
+package licenses
+
+import "github.com/anchore/syft/internal"
+
+// all of these taken from https://github.com/golang/pkgsite/blob/8996ff632abee854aef1b764ca0501f262f8f523/internal/licenses/licenses.go#L338
+// which unfortunately is not exported. But fortunately is under BSD-style license.
+
+var (
+	FileNames = []string{
+		"COPYING",
+		"COPYING.md",
+		"COPYING.markdown",
+		"COPYING.txt",
+		"LICENCE",
+		"LICENCE.md",
+		"LICENCE.markdown",
+		"LICENCE.txt",
+		"LICENSE",
+		"LICENSE.md",
+		"LICENSE.markdown",
+		"LICENSE.txt",
+		"LICENSE-2.0.txt",
+		"LICENCE-2.0.txt",
+		"LICENSE-APACHE",
+		"LICENCE-APACHE",
+		"LICENSE-APACHE-2.0.txt",
+		"LICENCE-APACHE-2.0.txt",
+		"LICENSE-MIT",
+		"LICENCE-MIT",
+		"LICENSE.MIT",
+		"LICENCE.MIT",
+		"LICENSE.code",
+		"LICENCE.code",
+		"LICENSE.docs",
+		"LICENCE.docs",
+		"LICENSE.rst",
+		"LICENCE.rst",
+		"MIT-LICENSE",
+		"MIT-LICENCE",
+		"MIT-LICENSE.md",
+		"MIT-LICENCE.md",
+		"MIT-LICENSE.markdown",
+		"MIT-LICENCE.markdown",
+		"MIT-LICENSE.txt",
+		"MIT-LICENCE.txt",
+		"MIT_LICENSE",
+		"MIT_LICENCE",
+		"UNLICENSE",
+		"UNLICENCE",
+	}
+
+	FileNameSet = internal.NewStringSet(FileNames...)
+)

internal/licenses/parser.go

@@ -0,0 +1,33 @@
+package licenses
+
+import (
+	"io"
+
+	"github.com/google/licensecheck"
+	"golang.org/x/exp/slices"
+)
+
+const (
+	coverageThreshold  = 75
+	unknownLicenseType = "UNKNOWN"
+)
+
+// Parse scans the contents of a license file to attempt to determine the type of license it is
+func Parse(reader io.Reader) (licenses []string, err error) {
+	contents, err := io.ReadAll(reader)
+	if err != nil {
+		return nil, err
+	}
+	cov := licensecheck.Scan(contents)
+
+	if cov.Percent < float64(coverageThreshold) {
+		licenses = append(licenses, unknownLicenseType)
+	}
+	for _, m := range cov.Match {
+		if slices.Contains(licenses, m.ID) {
+			continue
+		}
+		licenses = append(licenses, m.ID)
+	}
+	return
+}