An empty directory in the plugin directory makes load_all fail

[shym]

The dune-site plugin system expects all the entries of the plugin directory to be directories, each of which should contain a META file.
Unfortunately, we can end up with an empty directory there in a rather normal course of actions, after removing a package that provides a plugin.

Here is a small scenario that shows the problem. First, the following commands exhibit the intended behaviour:

$ opam install ortac-core
[...]

$ ortac
ortac: error: no plugin is available, please install at least one

$ opam install ortac-qcheck-stm
[...]

$ ortac
ortac: error: missing command
Usage: ortac [COMMAND] …
Try 'ortac --help' for more information.

But removing the second package breaks the first:

$ opam remove ortac-qcheck-stm
[...]

$ ortac
Fatal error: exception The plugin "qcheck-stm" can't be found in the search paths "%{prefix}%/lib/ortac-core/plugins".

where one would expect the initial behaviour (no plugin is available...).