populate: filter the program's sources by name

The filter for the program's sources took the paths it gets relative
to toString ./., which they needn't start with, e.g. if a symlink leads
to the directory, so nothing matched, and the build found no
Cargo.lock.  It goes by the names of what it leaves out now: the Nix
files, the docs, .gitignore, and cargo's target directory.

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