Handle a version with a dash in the prerelease tag

When parsing a constraint, before converting dashes to a range,
check if it is a valid version first. That way a version with multiple
dashes, like 1.0.0-12-abc123 isn't turned into a range of
1.0.0 to 12-abc123.

constraints_test.go

@@ -53,6 +53,12 @@ func TestParseConstraint(t *testing.T) {
 			includeMin: true,
 			includeMax: false,
 		}, false},
+		{"^1.1.0-12-abc123", rangeConstraint{
+			min:        Version{major: 1, minor: 1, patch: 0, pre: "12-abc123"},
+			max:        newV(2, 0, 0),
+			includeMin: true,
+			includeMax: false,
+		}, false},
 	}
 
 	for _, tc := range tests {
@@ -70,7 +76,7 @@ func TestParseConstraint(t *testing.T) {
 		}
 
 		if !constraintEq(tc.c, c) {
-			t.Errorf("Incorrect version found on %s", tc.in)
+			t.Errorf("%q produced constraint %q, but expected %q", tc.in, c, tc.c)
 		}
 	}
 }

parse.go

@@ -7,6 +7,9 @@ import (
 )
 
 func rewriteRange(i string) string {
+	if versionRegex.MatchString(i) {
+		return i
+	}
 	m := constraintRangeRegex.FindAllStringSubmatch(i, -1)
 	if m == nil {
 		return i