feat(node-path): append temp packages to $NODE_PATH

[knksmith57]

Fixes: zkat/npx#153
Ref: zkat/npx#160

This makes it so you can do npx -p lodash node -r lodash when lodash is not
already installed. One big downside of this patch is that because of the semantics
of that node feature, NODE_PATH will always be treated as a fallback. That is, if
you already have one version of lodash installed in a local npm project you're
currently in the directory of, you won't be able to override it with the
temporary install.

[knksmith57]

This PR cherry-picks @zkat's original commit (zkat/npx@df053f9) to land support for require()ing no-install dependencies (see zkat/npx#180).

There's a lot of value in landing this change as, among other things, it enables one-off use of programmatic APIs. Library developers often provide programmatic APIs far richer than those exposed by CLIs (assuming a CLI is even provided).

[knksmith57]

any interest in moving this forward, @isaacs?

please let me know if there's anything else I can do. thanks!

[darcyclarke]

@knksmith57 thanks for this! Unfortunately, the npx codebase has been pulled back into the npm/cli repo as it ships with npm by default. We'll be archiving this separate repo shortly; Feel free to direct any changes/improvements you want to see to npx/npm exec there.