Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libcmd/network-proxy.cc

@@ -0,0 +1,50 @@
+#include "network-proxy.hh"
+
+#include <algorithm>
+
+#include "environment-variables.hh"
+
+namespace nix {
+
+static const StringSet lowercaseVariables{"http_proxy", "https_proxy", "ftp_proxy", "all_proxy", "no_proxy"};
+
+static StringSet getAllVariables()
+{
+    StringSet variables = lowercaseVariables;
+    for (const auto & variable : lowercaseVariables) {
+        std::string upperVariable;
+        std::transform(
+            variable.begin(), variable.end(), upperVariable.begin(), [](unsigned char c) { return std::toupper(c); });
+        variables.insert(std::move(upperVariable));
+    }
+    return variables;
+}
+
+const StringSet networkProxyVariables = getAllVariables();
+
+static StringSet getExcludingNoProxyVariables()
+{
+    static const StringSet excludeVariables{"no_proxy", "NO_PROXY"};
+    StringSet variables;
+    std::set_difference(
+        networkProxyVariables.begin(),
+        networkProxyVariables.end(),
+        excludeVariables.begin(),
+        excludeVariables.end(),
+        std::inserter(variables, variables.begin()));
+    return variables;
+}
+
+static const StringSet excludingNoProxyVariables = getExcludingNoProxyVariables();
+
+bool haveNetworkProxyConnection()
+{
+    for (const auto & variable : excludingNoProxyVariables) {
+        if (getEnv(variable).has_value()) {
+            return true;
+        }
+    }
+    return false;
+}
+
+}