Move /src to /subprojects

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

subprojects/libutil/posix-source-accessor.hh

@@ -0,0 +1,72 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+struct SourcePath;
+
+/**
+ * A source accessor that uses the Unix filesystem.
+ */
+struct PosixSourceAccessor : virtual SourceAccessor
+{
+    /**
+     * Optional root path to prefix all operations into the native file
+     * system. This allows prepending funny things like `C:\` that
+     * `CanonPath` intentionally doesn't support.
+     */
+    const std::filesystem::path root;
+
+    PosixSourceAccessor();
+    PosixSourceAccessor(std::filesystem::path && root);
+
+    /**
+     * The most recent mtime seen by lstat(). This is a hack to
+     * support dumpPathAndGetMtime(). Should remove this eventually.
+     */
+    time_t mtime = 0;
+
+    void readFile(
+        const CanonPath & path,
+        Sink & sink,
+        std::function<void(uint64_t)> sizeCallback) override;
+
+    bool pathExists(const CanonPath & path) override;
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override;
+
+    DirEntries readDirectory(const CanonPath & path) override;
+
+    std::string readLink(const CanonPath & path) override;
+
+    std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override;
+
+    /**
+     * Create a `PosixSourceAccessor` and `CanonPath` corresponding to
+     * some native path.
+     *
+     * The `PosixSourceAccessor` is rooted as far up the tree as
+     * possible, (e.g. on Windows it could scoped to a drive like
+     * `C:\`). This allows more `..` parent accessing to work.
+     *
+     * See
+     * [`std::filesystem::path::root_path`](https://en.cppreference.com/w/cpp/filesystem/path/root_path)
+     * and
+     * [`std::filesystem::path::relative_path`](https://en.cppreference.com/w/cpp/filesystem/path/relative_path).
+     */
+    static SourcePath createAtRoot(const std::filesystem::path & path);
+
+private:
+
+    /**
+     * Throw an error if `path` or any of its ancestors are symlinks.
+     */
+    void assertNoSymlinks(CanonPath path);
+
+    std::optional<struct stat> cachedLstat(const CanonPath & path);
+
+    std::filesystem::path makeAbsPath(const CanonPath & path);
+};
+
+}