refactor: use native fs promise-based API instead of promisify & fix formatting

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

• there are few places where the callback version of native nodejs modules are used
• I found one place in which a string concatenation for file path was used when we could use the cross-platform version

What is the new behavior?

• replace those calls of fs. with their Promise-based API since they were introduced in nodejs v10.0.0
• replace "${foo}/${bar}" with path.join(foo, bar) for cross-platform compatibility sake
• add unit tests for PackageManagerFactory.find
• run npm run format

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[kamilmysliwiec]

lgtm