Address bug in remote CSS dependency handling

[rpkyle]

#53 introduced a get_package_mapping function to construct a map matching keys$package from the _dash-component-suites handler to a (currently loaded) R package which supplies the dependency (if present).

While JavaScript dependency handling appears to work as intended, the update was not tested against remote CSS dependencies. This should be resolved before asset serving is implemented.

The crux of the issue is that get_package_mapping currently requires that four pieces of information are available:

• a dependency name
• the dependency "package" name from the URL (which is generally ≠ to the R package name)
• a list of dependencies in htmltools format
• the R package name, registered within each element of the dependency list described above

A PR (#57) has been proposed to handle this potential issue, by asserting that dependencies have valid name, URL package, and R package information before constructing the package map.