Do not run pip to install Python plugin packages under uv or Poetry - #24951

`pulumi install` ran `python -m pip install <dir>` for each Python
plugin that is a buildable package, for every toolchain. A virtual
environment that uv creates does not contain pip, so a policy pack or a
component provider with `toolchain: uv` and a `[build-system]` table
failed with `No module named pip`.

Add `InstallPackage` to the `Toolchain` interface and call it before
`InstallDependencies`. The pip toolchain runs `pip install <dir>`,
because its `InstallDependencies` installs only `requirements.txt`. The
uv and Poetry toolchains do nothing, because `uv sync` and
`poetry install` install the project itself.

Fixes #24949