Remove validation of URLs passed to FileTransferRequest verbatim

CURL is not very strict about validation of URLs passed to it. We
should reflect this in our handling of URLs that we get from the user
in <nix/fetchurl.nix> or builtins.fetchurl. ValidURL was an attempt to
rectify this, but it turned out to be too strict. The only good way to
resolve this is to pass (in some cases) the user-provided string verbatim
to CURL. Other usages in libfetchers still benefit from using structured
ParsedURL and validation though.

nix store prefetch-file --name foo 'https://cdn.skypack.dev/big.js@^5.2.2'
error: 'https://cdn.skypack.dev/big.js@^5.2.2' is not a valid URL: leftover

(cherry picked from commit 47f427a172)

src/libutil/url.cc

@@ -434,7 +434,7 @@ bool isValidSchemeName(std::string_view s)
     return std::regex_match(s.begin(), s.end(), regex, std::regex_constants::match_default);
 }
 
-std::ostream & operator<<(std::ostream & os, const ValidURL & url)
+std::ostream & operator<<(std::ostream & os, const VerbatimURL & url)
 {
     os << url.to_string();
     return os;