Composer installs path repositories as symlinks by default, which causes Magento's template engine to fail when resolving .phtml files. Setting `COMPOSER_MIRROR_PATH_REPOS=1` on all composer install steps forces a copy instead, matching how the package would be installed from Packagist in production.
Adds a .phtml template and an integration test to the demo package that
renders it via Magento's template engine. Without COMPOSER_MIRROR_PATH_REPOS=1
the path repo is installed as a symlink and the test fails; with mirroring
it passes.
Closes#217
Damien Retzinger