remove node v6 and NPM support

[smanders]

externpro has some cmake to support webpro (see issue #115)

some cmake functions were migrated from webpro to externpro as part of that work (e45224a)

• wpPatch (from webpro) integrated into xpPatch with NPM_INSTALL option
• wpBuildNpmModule renamed to xpBuildNpmModule and slightly modified (NPM_FLAGS)

webpro is migrating away from NPM to yarn... and it makes sense for webpro to be the home for yarn-specific cmake (wpBuildYarnModule, for example) -- and the webpro projects aren't using the NPM_INSTALL option any longer, either...

so this issue is to track removing the NPM support that's in externpro which was added with the commit referenced above

[smanders]

we also copy NPM into the externpro installer and dedupe it's directory structure to make it all fit in the installer, etc... this should also be able to be removed https://github.com/smanders/externpro/blob/18.04.1/projects/node.cmake#L78-L108 -- but maybe not until we're only building node version 8 and higher... version 8 is what's forced us to migrate from NPM to yarn in webpro and we're still using NPM with our current build of node version 6

[smanders]

and remove NODE_NPM from the use script https://github.com/smanders/externpro/blob/18.04.1/projects/use/usexp-node-config.cmake#L27

[smanders]

removing node v6 will also help us step closer to supporting Visual Studio 2017 - when attempting to build node v6 with VS2017 there is a message "Looking for Visual Studio 2015 -- Failed to find Visual Studio installation" #189

[smanders]

completed with commits referenced above (and below -- remove from projects/README.md)