Move /src to /subprojects

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

subprojects/libutil/users.cc

@@ -0,0 +1,100 @@
+#include "util.hh"
+#include "users.hh"
+#include "environment-variables.hh"
+#include "file-system.hh"
+
+namespace nix {
+
+Path getCacheDir()
+{
+    auto dir = getEnv("NIX_CACHE_HOME");
+    if (dir) {
+        return *dir;
+    } else {
+        auto xdgDir = getEnv("XDG_CACHE_HOME");
+        if (xdgDir) {
+            return *xdgDir + "/nix";
+        } else {
+            return getHome() + "/.cache/nix";
+        }
+    }
+}
+
+
+Path getConfigDir()
+{
+    auto dir = getEnv("NIX_CONFIG_HOME");
+    if (dir) {
+        return *dir;
+    } else {
+        auto xdgDir = getEnv("XDG_CONFIG_HOME");
+        if (xdgDir) {
+            return *xdgDir + "/nix";
+        } else {
+            return getHome() + "/.config/nix";
+        }
+    }
+}
+
+std::vector<Path> getConfigDirs()
+{
+    Path configHome = getConfigDir();
+    auto configDirs = getEnv("XDG_CONFIG_DIRS").value_or("/etc/xdg");
+    std::vector<Path> result = tokenizeString<std::vector<std::string>>(configDirs, ":");
+    for (auto& p : result) {
+        p += "/nix";
+    }
+    result.insert(result.begin(), configHome);
+    return result;
+}
+
+
+Path getDataDir()
+{
+    auto dir = getEnv("NIX_DATA_HOME");
+    if (dir) {
+        return *dir;
+    } else {
+        auto xdgDir = getEnv("XDG_DATA_HOME");
+        if (xdgDir) {
+            return *xdgDir + "/nix";
+        } else {
+            return getHome() + "/.local/share/nix";
+        }
+    }
+}
+
+Path getStateDir()
+{
+    auto dir = getEnv("NIX_STATE_HOME");
+    if (dir) {
+        return *dir;
+    } else {
+        auto xdgDir = getEnv("XDG_STATE_HOME");
+        if (xdgDir) {
+            return *xdgDir + "/nix";
+        } else {
+            return getHome() + "/.local/state/nix";
+        }
+    }
+}
+
+Path createNixStateDir()
+{
+    Path dir = getStateDir();
+    createDirs(dir);
+    return dir;
+}
+
+
+std::string expandTilde(std::string_view path)
+{
+    // TODO: expand ~user ?
+    auto tilde = path.substr(0, 2);
+    if (tilde == "~/" || tilde == "~")
+        return getHome() + std::string(path.substr(1));
+    else
+        return std::string(path);
+}
+
+}