There are some downsides to this features:

 - Submodules are not cached (unlike the root repo),
 - Full checkouts are created in a temporary directory.

This fixes fetching repositories with no submodules and also cleans up
.git files in checkouts.

Some platforms seem to still require linking with stdc++fs to enable
STL std::filesystem support.

…ue with submodules

Before this change it would be false for all evaluations but the first.
Now it follows the input argument (as it should).

The .link file is used as a lock, so I think we should put the
"submodule" attribute in there since turning on submodules creates a new
.link file path.

Major bugfix for the submodules = true code path.

TODO: Add tests.

Due to fetchGit not checking if rev is an ancestor of ref (there is even
a FIXME comment about it in the code), the cache repo might not have the
ref even though it has the rev. This doesn't matter when submodule =
false, but the submodule = true code blows up because it tries to fetch
the (missing) ref from the cache repo.

Fix this in the simplest way possible: fetch all refs from the local
cache repo when submodules = true.

TODO: Add tests.

Using std::filesystem means also having to link with -lstdc++fs on
some platforms and it's hard to discover for what platforms this is
needed. As all the functionality is already implemented as utilities,
use those instead.