Add installer events

| Q             | A
| ------------- | ------
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3116
| License       | MIT

This PR adds 2 new events:
- `Composer\Installer\InstallerEvents`::`PRE_SOLVE_DEPENDENCIES`
- `Composer\Installer\InstallerEvents`::`POST_SOLVE_DEPENDENCIES`