Type annotations improvements

[nsoranzo]

Also:

• Make InstallationTarget a runtime-checkable protocol
• Reuseinstalled_repository_manager attribute instead of instantiating a InstalledRepositoryManager in several places.
• Fix typos
• Proper subclassing of ModelManager generic class

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[nsoranzo]

Test failures unrelated.

[nsoranzo]

Pushed a couple of more changes. Test failures still unrelated.

[davelopez]

Thank you @nsoranzo for the nice type improvements!