[FEATURE]: support npm subpath imports for custom provider npm entries #16586

• I have verified this feature I'm about to request hasn't been suggested before.

TL;DR

Please support npm subpath imports in custom provider npm entries.

Wanted behavior:

• install package root
• import full requested specifier

Example:

• pkg -> install pkg, import pkg
• pkg/subpath -> install pkg, import pkg/subpath
• @scope/pkg/subpath -> install @scope/pkg, import @scope/pkg/subpath

Describe the enhancement you want to request

Custom provider npm values currently seem to work well for plain package specifiers such as pkg and @scope/pkg, but not for standard npm subpath imports such as pkg/subpath and @scope/pkg/subpath.

This matters because a provider package may reasonably want to expose multiple entrypoints while keeping its root export unchanged. For example:

• a root entry for general usage
• a subpath export for a more specialized integration surface, such as OpenCode-specific optimization or API/subscription path branching

Expected behavior for non-file:// specifiers:

• parse configured npm as an import specifier
• install only the root package
• import the full requested specifier

This keeps plain package behavior unchanged while adding support for normal npm subpath exports.

Suggested tests:

• plain package custom provider still loads
• scoped plain package custom provider still loads
• package subpath custom provider loads
• scoped package subpath custom provider loads
• invalid subpath fails cleanly
• pkg and pkg/subpath do not collide in cache or loader state

Note: I used AI assistance during investigation and English drafting because English is not my first language.