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.cc

@@ -0,0 +1,54 @@
+#include "memory-input-accessor.hh"
+
+namespace nix {
+
+struct MemoryInputAccessorImpl : MemoryInputAccessor
+{
+    std::map<CanonPath, std::string> files;
+
+    std::string readFile(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        if (i == files.end())
+            throw Error("file '%s' does not exist", path);
+        return i->second;
+    }
+
+    bool pathExists(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        return i != files.end();
+    }
+
+    Stat lstat(const CanonPath & path) override
+    {
+        auto i = files.find(path);
+        if (i != files.end())
+            return Stat { .type = tRegular, .isExecutable = false };
+        throw Error("file '%s' does not exist", path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return {};
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        throw UnimplementedError("MemoryInputAccessor::readLink");
+    }
+
+    SourcePath addFile(CanonPath path, std::string && contents) override
+    {
+        files.emplace(path, std::move(contents));
+
+        return {ref(shared_from_this()), std::move(path)};
+    }
+};
+
+ref<MemoryInputAccessor> makeMemoryInputAccessor()
+{
+    return make_ref<MemoryInputAccessorImpl>();
+}
+
+}