Improve the error message when trying to use a `build-scripts` package as a regular requirement

[valgur]

It currently fails on an assert with no further context provided in the stack trace. There might be a better place to do this check, though.

Also, might want to add a check for PackageType.PYTHON as well.

Changelog: Fix: Improve the error message when trying to use a build-scripts package as a regular requirement Docs: Omit

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