Extended project files (conditionals and imports) (#7783)

Extended project files (conditionals and imports) (#7783)

* initial parser pass

* first compiling pass

* get more stuff sort of working

* conditional parsing actually works

* error cleanup and downloads

* thread through http transport

* fix merge

* better errors and use extended project parsing uniformly

* elif support, maybe?

* fix outdated cmd, add tests, docs

* fix docs

* use legacyReadFields parser

* changelog

* cyclical import detection

* fix shadowing

* add missing file

* finish merge

* fix outstanding merge issue

* use existing config available when checking for compiler for package flags

* review comments

* add missing test file

* Update pr-7783

Co-authored-by: Gershom Bazerman <gershom@arista.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>