InstallableFlake::toDerivedPaths(): Support paths and store paths

This makes 'nix build' work on paths (which will be copied to the
store) and store paths (returned as is). E.g. the following flake
output attributes can be built using 'nix build .#foo':

  foo = ./src;
  foo = self.outPath;
  foo = builtins.fetchTarball { ... };
  foo = (builtins.fetchTree { .. }).outPath;
  foo = builtins.fetchTree { .. } + "/README.md";
  foo = builtins.storePath /nix/store/...;

Note that this is potentially risky, e.g.

  foo = /.;

will cause Nix to try to copy the entire file system to the store.

What doesn't work yet:

  foo = self;
  foo = builtins.fetchTree { .. };

because we don't handle attrsets with an outPath attribute in it yet,
and

  foo = builtins.storePath /nix/store/.../README.md;

since result symlinks have to point to a store path currently (rather
than a file inside a store path).

Fixes #7417.

src/libfetchers/input-accessor.hh

@@ -66,7 +66,7 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
     StorePath fetchToStore(
         ref<Store> store,
         const CanonPath & path,
-        std::string_view name,
+        std::string_view name = "source",
         PathFilter * filter = nullptr,
         RepairFlag repair = NoRepair);
 
@@ -151,7 +151,7 @@ struct SourcePath
 
     StorePath fetchToStore(
         ref<Store> store,
-        std::string_view name,
+        std::string_view name = "source",
         PathFilter * filter = nullptr,
         RepairFlag repair = NoRepair) const;