When someone explicitly opts into using the node_modules dir via
`--node-modules-dir` or setting `"nodeModulesDir": true` in the
deno.json file, we should eagerly ensure a top level package.json
install is done on startup. This was initially always done when we added
package.json support and a package.json was auto-discovered, but scaled
it back to be lazily done when a bare specifier matched an entry in the
package.json because of how disruptive it was for people using Deno
scripts in Node projects. That said, it does not make sense for someone
to opt-into having deno control and use their node_modules directory and
not want a package.json install to occur. If such a rare scenario
exists, the `DENO_NO_PACKAGE_JSON=1` environment variable can be set.
Ideally, we would only ever use a node_modules directory with this
explicit opt-in so everything is very clear, but we still have this
automatic scenario when there's a package.json in order to make more
node projects work out of the box.
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