LibWeb: Move Fetch infra into the Web::Fetch::Infrastructure namespace

The Fetch spec unfortunately will cause a name clash between the Request
concept and the Request JS object - both cannot live in the Web::Fetch
namespace, and WrapperGenerator generally assumes `Web::<Name>` for
things living in the `<Name>/` subdirectory, so let's instead move infra
code into its own namespace - it already sits in a (sub-)subdirectory
anyway.

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp

@@ -16,7 +16,7 @@
 #include <LibWeb/Infra/ByteSequences.h>
 #include <LibWeb/MimeSniff/MimeType.h>
 
-namespace Web::Fetch {
+namespace Web::Fetch::Infrastructure {
 
 template<typename T>
 requires(IsSameIgnoringCV<T, u8>) struct CaseInsensitiveBytesTraits : public Traits<Span<T>> {