This improves the metadata generation, previously it would take any
"license" entry from the nuspec, and tried to match it to an spdx ID from
"lib.licenses".
Sometimes however licenses are provided in plain-text, which we
obviously cannot cleanly resolve. This resulted in in useless information
("LICENSE.txt") being written to "meta.license".
Uses fsharp interactive (fsi) to run fsx script
Expose mkNugetSource and mkNugetDeps functions
Use them in writers.writeFSharp and buildDotnetModule
Add tests