Runs pre-gen command before validating project

[simonbs]

This PR updates the order of operations in the generate command to run the pre-gen command before project validation.

When running the generate command, the validation step fails if a target's sources list contains directories that don't exist yet. This is an issue when these directories are meant to be created by a pre-gen command.

Consider the following project specification:

name: Example
options:
  preGenCommand: ./scripts/generate-sources.sh
targets:
  Example:
    scheme: {}
    deploymentTarget: '16.0'
    platform: iOS
    type: application
    sources:
    - path: Example
    - path: GeneratedSources

The pre-gen command generates Swift source files into the GeneratedSources folder, which is in .gitignore. Without the changes in this PR, running generate results in an error:

Spec validation error: Target "Example" has a missing source directory "/Users/simonbs/Developer/Example/GeneratedSources"

With the changes in this PR, the pre-gen command is executed before the project is validated, meaning that GeneratedSources is present during validation. This prevents the error and causes project generation to succeed.

[yonaskolb]

Makes sense, thanks @simonbs