Do not treat build scripts as full package.

ScanCode Toolkit should not "assemble" some less traveled and less complete package data file formats such as autotools: this means that we should never create a full top level package for these. This includes several "build" tools.

- These could be treated as something entirely different from a package or package data: a set of build instructions?
- Or we could keep these as mere file-level package-like data and not create top level package. This would mean moving these to subclasses of "NonAssemblableDatafileHandler".

For instance with: https://www.antlr2.org/download/antlr-2.7.7.tar.gz I get an pkg:autotools/antlr-2.7.5 which is not useful. Autotools "configure" script is often under an fsf-free license and this is 99% of the time NOT the license of the package.

We should extend this to all types defined in packagedcode/build.py