Fix #2046 - Dependencies specified with a path ignore optional settings

[shoo]

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[CyberShadow]

@shoo How do I get the old behavior?

I would like to use a package with a path if it exists at that path, regardless if it's specified in some dub.selections.json file.

That way we can use autoconf-like autodetection to build features which use only those dependencies that are installed on the local system. But after this change, I don't see how this is possible. With optional=true default=true, Dub doesn't use the package even when it's present; if I also add it to dub.selections.json, then it tries to use it even if it's absent.