populate: name inputs of generated flakes validly

Generated flakes used source names as input names, but Nix accepts fewer
names there than populate does, e.g. not alt-1.0 or self.  Following such
a source failed late, at locking, with an error mentioning neither krops
nor the source.  Names Nix doesn't accept get replaced by ones derived
from them.

An edge case follows a source named alt-1.0.

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