`--single` should not require a name for the target

[schveiguy]

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When building with the --single mode (i.e. a single D file with the dub recipe in the comments), dub requires a name directive. However, dmd does not require this, and will name the target executable after the filename. Dub should do the same thing.

Basically, for dub --single, default the name of the target in foo.d to foo.