Glob pattern in nuspec files doesn't work

A nuspec file like:
```

MyPackg
2.3.0
MyPackg
Big Technologies
Big Technologies
false
MyPackg - Retail
Retail front-end.

Copyright 2019
MyPackg

```

produces an error:
`"Nuget: cannot build c:\pkg.nuspec. Missing files: c:\package\**\*.*"`

No source file, test, or entry point is named in the issue. Reproduce the failure with the provided nuspec and check how the package builder handles the c:\package\**\*.* source pattern; done means the glob resolves matching files instead of reporting the literal pattern as missing.

Written by the indexing model from the issue text.