Allow uppercase package name in package query expressions

Fixes: #636

Before this fix, aptly was always treating strings starting with
uppercase letter as field name, which was breaking package queries
like `VMware-Horizon-Client_4.5.0_all`.

Now aptly accepts only fields which don't contain underscore, and
everything else would be parsed as package reference.

query/syntax.go

@@ -135,7 +135,7 @@ func (p *parser) D() deb.PackageQuery {
 	operator, value := p.Condition()
 
 	r, _ := utf8.DecodeRuneInString(field)
-	if strings.HasPrefix(field, "$") || unicode.IsUpper(r) {
+	if strings.HasPrefix(field, "$") || (unicode.IsUpper(r) && !strings.ContainsRune(field, '_')) {
 		// special field or regular field
 		q := &deb.FieldQuery{Field: field, Relation: operatorToRelation(operator), Value: value}
 		if q.Relation == deb.VersionRegexp {