module: speed up package.json parsing

If the package.json does not contain the string '"main"', skip parsing
it to JSON.

Note that this changes the behavior of the module loader in the presence
of package.json files that don't contain legal JSON.  Such files used to
throw an exception but now they are simply ignored unless they contain a
"main" property.

To me, that seems like a good trade-off: I observe a 25% reduction in
start-up time on a medium-sized application[0].

[0] https://github.com/strongloop/sls-sample-app

PR-URL: https://github.com/nodejs/node/pull/15767


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>