Add support for unnamed projects

Sometimes (e.g., in bpkg configuration) we don't have a project name.
In fact, it is not really a project; it can never be referenced in an
import directive.

So we now have a notion of an unnamed project. Such a project should
still have the 'project' variable set first thing in bootstrap.build
but its value should be empty.

Note that we can still amalgamate such projects just liked named ones.

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