Remove std::string alias (for real this time)

Also use std::string_view in a few more places.

src/libstore/builtins/fetchurl.cc

@@ -16,7 +16,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         writeFile(settings.netrcFile, netrcData, 0600);
     }
 
-    auto getAttr = [&](const string & name) {
+    auto getAttr = [&](const std::string & name) {
         auto i = drv.env.find(name);
         if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
         return i->second;