Changes the name of generated rules to be valid in more cases

[mattnathan]

In addition if you are outputting the generated files into a folder that
looks like the npm package name then omit the prefix part to simplify
importing in bazel.

For example if you npm package is @vue/cli and your output dir is
~/myrepo/vue/cli then the generated rule name will now be cli which
will allow importing like //myrepo/vue/cli. If the output dir doesn't
match the package name then any / will be replaced by '_' in the rule
name.

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