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

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

src/libstore/path-with-outputs.cc

@@ -49,9 +49,9 @@ std::pair<std::string_view, StringSet> parsePathWithOutputs(std::string_view s)
 {
     size_t n = s.find("!");
     return n == s.npos
-        ? std::make_pair(s, std::set<string>())
+        ? std::make_pair(s, std::set<std::string>())
         : std::make_pair(((std::string_view) s).substr(0, n),
-            tokenizeString<std::set<string>>(((std::string_view) s).substr(n + 1), ","));
+            tokenizeString<std::set<std::string>>(((std::string_view) s).substr(n + 1), ","));
 }