libutil: Refactor percentDecode,percentEncode to use Boost.URL

The myriad of hand-rolled URL parsing and validation code
is a constant source of problems. Regexes are not a great way
of writing parsers and there's a history of getting them wrong.
Boost.URL is a good library we can outsource most of the heavy
lifting to.

src/libutil-tests/url.cc

@@ -1,5 +1,7 @@
 #include "nix/util/url.hh"
+#include "nix/util/tests/gmock-matchers.hh"
 #include <gtest/gtest.h>
+#include <gmock/gmock.h>
 
 namespace nix {
 
@@ -289,6 +291,14 @@ TEST(percentDecode, trailingPercent)
     ASSERT_EQ(d, s);
 }
 
+TEST(percentDecode, incompleteEncoding)
+{
+    ASSERT_THAT(
+        []() { percentDecode("%1"); },
+        ::testing::ThrowsMessage<BadURL>(
+            testing::HasSubstrIgnoreANSIMatcher("error: invalid URI parameter '%1': incomplete pct-encoding")));
+}
+
 /* ----------------------------------------------------------------------------
  * percentEncode
  * --------------------------------------------------------------------------*/