Feature request: read `packageManager` and `devEngines` fields from outside of package.json

[zanminkian]

Use case:

When developing some projects which reject to add packageManager and devEngines fields to its package.json, it would be not easy to use correct pacakge manager and package manager version, even though I've enabled corepack.

if corepack reads packageManager and devEngines fields from outside of package.json like .corepack.env, it would be convenient.

[aduh95]

You should expose your use-case in #668, and explain why COREPACK_ENABLE_AUTO_PIN=0 is not enough