versionfmt/dpkg: remove leading digit requirement

This is not strictly a requirement and affects some tracked Alpine Linux
packages.

ext/versionfmt/dpkg/parser.go

@@ -96,10 +96,6 @@ func newVersion(str string) (version, error) {
 		return version{}, errors.New("No version")
 	}
 
-	if !unicode.IsDigit(rune(v.version[0])) {
-		return version{}, errors.New("version does not start with digit")
-	}
-
 	for i := 0; i < len(v.version); i = i + 1 {
 		r := rune(v.version[i])
 		if !unicode.IsDigit(r) && !unicode.IsLetter(r) && !containsRune(versionAllowedSymbols, r) {

ext/versionfmt/dpkg/parser_test.go

@@ -70,8 +70,10 @@ func TestParse(t *testing.T) {
 		// Test invalid characters in epoch
 		{"a:0-0", version{}, true},
 		{"A:0-0", version{}, true},
-		// Test version not starting with a digit
-		{"0:abc3-0", version{}, true},
+		// Test version not starting with a digit.
+		// While recommended by the specification, this is not strictly required and
+		// at least one vulnerable Alpine package deviates from this scheme.
+		{"0:abc3-0", version{epoch: 0, version: "abc3", revision: "0"}, false},
 	}
 	for _, c := range cases {
 		v, err := newVersion(c.str)