public async Task <InstalledPackage> Install(ISource source, PackageSpec package)
{
var pdir = GetPackageDir(package);
if (Directory.Exists(pdir))
{
throw new Exception("Package already installed.");
}
FS.EnsureDirectoryExists(pdir);
var nupkgDest = GetNupkgPath(package);
FS.EnsureParentDirectoryExists(nupkgDest);
await source.Download(package, nupkgDest);
ZipFile.ExtractToDirectory(nupkgDest, pdir);
return(new InstalledPackage(this, package));
}