Allow creation of "pre-install" script

Issue Kind

Brand new capability

Description

It would be great for Poetry to allow developers to implement a "pre-install" script. As of now, the scripts of Poetry are available after running "poetry install" once. Sometimes, users will need to run a script before running "poetry install", as the example below shows.

Impact

In our company, we use local packages references like the following in a pyproject.toml:
my_local_package = { path = "./packages/my_local_package", develop = true }
But our local package is not actually in the given path : we have to copy it beforehand. We did this so that production packages are decoupled from other parts of the code and only code that is being actively used by our production packages are on our docker images.
So we have something that looks like this:

• root_of_our_project
  • packages
    • local_package_A
    • local_package_B
    • local_package_C
  • production
    • production_package_A
      • src
        • <some_code_files_that_use_local_packages_A_and_C>
      • packages
        • local_package_A
        • local_package_C
    • production_package_B
      • src
        • <some_code_files_that_use_local_package_B>
      • packages
        • local_package_B

Therefore, a step that we have to do before doing "poetry install" on our production packages is to copy our local packages in the related "packages" folders via bash scripts.

It would be great to add a "pre-install" script implementation system so that we could just run "poetry install" and the copy of packages would be done under the hood before actually installing.

Workarounds

We currently run a bash file that will do the copy before doing "poetry install".

The issue body describes a proposed pre-install script capability and the current bash workaround for copying local packages. Start by reviewing Poetry's existing install and script behavior, then define how a pre-install script would be configured and invoked; done should allow the package-copy step to run before dependency installation.