Fix make-cjs-components script to work without mjs files being built first

[dpvc]

This PR resolves the problem where the webpacking failed when building the cjs bundle without having first build the mjs files. It was due to the webpack.common.cjs file using require.resolve() command to get full path names; since webpack.common.cjs is in the components directory, it was using the main package.json file, and so the pseudo-packages like #js were resolving to the mathjax-full/mjs direcrtory rather than the mathjax-full/cjs directory.

This resolves the problem by moving the fullPath() function to a separate fullpath.cjs file in the components/mjs and components/cjs directories, so that the cjs file will use the components/cjs/package.json mappings instead.

It also adds a test to the link:full script to only make the link if it is not already there, so it is safe to run the script more than once.

[zorkow]

lgtm.