Check for undefined libPath

When specifying a (local) package with no package.js libPath is undefined. This results in an exception:

    fs.js:561
    return binding.symlink(preprocessSymlinkDestination(destination, type),
        ^
        TypeError: dest path must be a string
    at Object.fs.symlinkSync (fs.js:561:18)
    at Package.installInto (/usr/local/lib/node_modules/meteorite/lib/dependencies/package.js:118:10)
    at LocalSource.fetch (/usr/local/lib/node_modules/meteorite/lib/sources/local.js:25:3)
    at Package.fetch (/usr/local/lib/node_modules/meteorite/lib/dependencies/package.js:79:15)
    at Package.installInto (/usr/local/lib/node_modules/meteorite/lib/dependencies/package.js:100:8)
    at Dependencies.installInto.installStep (/usr/local/lib/node_modules/meteorite/lib/dependencies/dependencies.js:62:22)
    at Dependencies.installInto (/usr/local/lib/node_modules/meteorite/lib/dependencies/dependencies.js:64:3)
    at Project.install (/usr/local/lib/node_modules/meteorite/lib/project.js:140:23)
    at Project.fetch (/usr/local/lib/node_modules/meteorite/lib/project.js:109:9)
    at Resolver.resolve (/usr/local/lib/node_modules/meteorite/lib/dependencies/resolver.js:47:5)

This PR just checks for undefined to prevent this exception. The installation of the package will fail silently then.