Support remotely loading npm modules

[jtq]

Rework module path/URL resolution code, so that when relativePath is:

relativePath	Resulting URL
"./token"	baseUrl+"/token"
"../token"	Parent of basePath+"/token"
/^[a-z][a-z0-9+\-\.]*:/i	relativePath (absolute URL)
/[^\._][a-z\@]*/	Resolve URL of published NPM module, and see if we can load it live from NPM or its public repo. Won't work for all modules, but should work well enough for simple pure-JS packages without install hooks