Move /src to /subprojects

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

subprojects/libfetchers/fetch-to-store.cc

@@ -0,0 +1,53 @@
+#include "fetch-to-store.hh"
+#include "fetchers.hh"
+#include "cache.hh"
+
+namespace nix {
+
+StorePath fetchToStore(
+    Store & store,
+    const SourcePath & path,
+    FetchMode mode,
+    std::string_view name,
+    ContentAddressMethod method,
+    PathFilter * filter,
+    RepairFlag repair)
+{
+    // FIXME: add an optimisation for the case where the accessor is
+    // a `PosixSourceAccessor` pointing to a store path.
+
+    std::optional<fetchers::Cache::Key> cacheKey;
+
+    if (!filter && path.accessor->fingerprint) {
+        cacheKey = fetchers::Cache::Key{"fetchToStore", {
+            {"name", std::string{name}},
+            {"fingerprint", *path.accessor->fingerprint},
+            {"method", std::string{method.render()}},
+            {"path", path.path.abs()}
+        }};
+        if (auto res = fetchers::getCache()->lookupStorePath(*cacheKey, store)) {
+            debug("store path cache hit for '%s'", path);
+            return res->storePath;
+        }
+    } else
+        debug("source path '%s' is uncacheable", path);
+
+    Activity act(*logger, lvlChatty, actUnknown,
+        fmt(mode == FetchMode::DryRun ? "hashing '%s'" : "copying '%s' to the store", path));
+
+    auto filter2 = filter ? *filter : defaultPathFilter;
+
+    auto storePath =
+        mode == FetchMode::DryRun
+        ? store.computeStorePath(
+            name, path, method, HashAlgorithm::SHA256, {}, filter2).first
+        : store.addToStore(
+            name, path, method, HashAlgorithm::SHA256, {}, filter2, repair);
+
+    if (cacheKey && mode == FetchMode::Copy)
+        fetchers::getCache()->upsert(*cacheKey, store, {}, storePath);
+
+    return storePath;
+}
+
+}