Introduce MemoryInputAccessor and use it for corepkgs

MemoryInputAccessor is an in-memory virtual filesystem that returns
files like <nix/fetchurl.nix>. This removes the need for special hacks
to handle those files.

src/libfetchers/memory-input-accessor.hh

@@ -0,0 +1,15 @@
+#include "input-accessor.hh"
+
+namespace nix {
+
+/**
+ * An input accessor for an in-memory file system.
+ */
+struct MemoryInputAccessor : InputAccessor
+{
+    virtual SourcePath addFile(CanonPath path, std::string && contents) = 0;
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor();
+
+}