what about language compiler-plugins and extensions?
for example, in php/composer, they are checked on install-time. see https://thomasventurini.com/articles/require-php-extensions-for-your-project-through-composer/

In the case of PHP, are those really "virtual"? I'm not too familiar with the ecosystem, but they feel like a concrete set of packages needed by the php interpreter, so they should not be expressed using the proposed type in this PR.

In the case of PHP, are those really "virtual"? I'm not too familiar with the ecosystem, but they feel like a concrete set of packages needed by the php interpreter, so they should not be expressed using the proposed type in this PR.

Theybare virtual. The extensions arr more of a contract to make certain interfaces available, the actual implementation is irrelevant.

These are actual virtual packages, just asbthe language compilers/interpreters/implementation.
Since there are multiple php langauge implementations (zend, hhvm, phalanger,...) each could have their own implementation of the individual lanuage extensions.

I see, thank for the details. I guess something along the lines of compiler-ext or language-ext? And now that I see you mention interpreter, I wonder if this should be added (e.g. CPython vs PyPy), but also I recognize that it might be tricky to draw the line between compiler and interpreter in some edge cases 😬

These are questions that should be discussed in the ticket, not in a proposed solution ;)