submodule: require the submodule path to contain directories only

Submodules are stored in subdirectories of their superproject. When
these subdirectories have been replaced with symlinks by a malicious
actor, all kinds of mayhem can be caused.

This _should_ not be possible, but many CVEs in the past showed that
_when_ possible, it allows attackers to slip in code that gets executed
during, say, a `git clone --recursive` operation.

Let's add some defense-in-depth to disallow submodule paths to have
anything except directories in them.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

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