Factor out MemorySourceAccessor, implement missing features

The new `MemorySourceAccessor` rather than being a slightly lossy flat
map is a complete in-memory model of file system objects.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libutil/memory-source-accessor.hh

@@ -0,0 +1,74 @@
+#include "source-accessor.hh"
+#include "variant-wrapper.hh"
+
+namespace nix {
+
+/**
+ * An source accessor for an in-memory file system.
+ */
+struct MemorySourceAccessor : virtual SourceAccessor
+{
+    /**
+     * In addition to being part of the implementation of
+     * `MemorySourceAccessor`, this has a side benefit of nicely
+     * defining what a "file system object" is in Nix.
+     */
+    struct File {
+        struct Regular {
+            bool executable = false;
+            std::string contents;
+
+            GENERATE_CMP(Regular, me->executable, me->contents);
+        };
+
+        struct Directory {
+            using Name = std::string;
+
+            std::map<Name, File, std::less<>> contents;
+
+            GENERATE_CMP(Directory, me->contents);
+        };
+
+        struct Symlink {
+            std::string target;
+
+            GENERATE_CMP(Symlink, me->target);
+        };
+
+        using Raw = std::variant<Regular, Directory, Symlink>;
+        Raw raw;
+
+        MAKE_WRAPPER_CONSTRUCTOR(File);
+
+        GENERATE_CMP(File, me->raw);
+
+        Stat lstat() const;
+    };
+
+    File root { File::Directory {} };
+
+    GENERATE_CMP(MemorySourceAccessor, me->root);
+
+    std::string readFile(const CanonPath & path) 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;
+
+    /**
+     * @param create If present, create this file and any parent directories
+     * that are needed.
+     *
+     * Return null if
+     *
+     * - `create = false`: File does not exist.
+     *
+     * - `create = true`: some parent file was not a dir, so couldn't
+     *   look/create inside.
+     */
+    File * open(const CanonPath & path, std::optional<File> create);
+
+    CanonPath addFile(CanonPath path, std::string && contents);
+};
+
+}