Here's a suggestion, do you think it could be this simple? If you write a package-manager, don't execute any of the downloaded code in an automated fashion. No hooks, no build-time metaprograms. If your language _needs_ metaprogramming to function, it's a huge secondary issue that I don't know how to solve. You can try to make a meta-program annotation which disables side-effects, but the metaprogram ultimately must write memory which expands the amount of code generated. All risk is introduced when the person downloading cannot preview the content in a safe place.
Unfortunately it doesn't solve the problem because malicious code can still end up compiled into the user's program which they will promptly execute, possibly in production...
Supply chain attacks are not a problem that can be solved by a single silver bullet, however the biggest benefit comes from a combination of minimum release age + fresh 2FA required for every publish + automated scanning. This makes it considerably more difficult to pull off a supply chain attack and should be the baseline security for all package managers.
The only safe solution is to review all the code's changes (or trust someone else to do it).
Forbidding compile/build-time shenaningans is trivially bypassable and has already been bypassed in the NPM ecosystem by just making the library code itself (not the build scripts) malicious - eg. do the bad thing when the code is loaded/tested, assuming the language has static constructors.