Add dynamic interpolation for package paths

It is extremely common and even recommended for Atmel AVR chips, when uploading via a programmer, to reference the path of a tool via e.g.

upload_flags =
    -C
    ; use "tool-avrdude-megaavr" for the atmelmegaavr platform
    ${platformio.packages_dir}/tool-avrdude/avrdude.conf

However, this is extremely fragile and non-reproducable, because sometimes the "tool-avrdude" that the project uses is actually in tool-avrdude@1.70100.0. Or there might be a case where platform_packages is used with a e.g. symlink:// so that the tool isn't even in ${platformio.packages_dir}.

See e.g. https://community.platformio.org/t/unable-to-upload-to-attiny814

This leads to frustration when users have to search for the errors in platformio.ini directives that the official documentation gives them. PlatformIO has no builtin way to reference the actual path used for a certain package.

Of course, one can write an extra script and use

Import("env")
platform = env.PioPlatform()
path = platform.get_package_dir("tool-avrdude")

to get the actual path, but that's very cumbersome. PlatformIO should have an interpolation syntax of maybe

${packages.tool-avrdude}

that fills in the actual path for the package.

Start with the package lookup entry point env.PioPlatform().get_package_dir(...) and the platformio.ini interpolation use described in the issue. Done means configuration such as upload_flags can resolve the actual package path, including versioned or symlink:// platform_packages, without an extra script.