fix(sync-types): map import-map aliases that point at a jsr subpath
An alias like `"$std/fs/walk.ts": "jsr:@std/fs@1/walk"` only had its scheme
key (`jsr:@std/fs@1/walk`) mapped, not the alias the source actually imports.
The alias key was emitted only for bare-package aliases (empty subpath). Emit
it whenever the entry came from an alias, so subpath-targeting aliases resolve.
dotcom 26 -> 25, fresh 124 -> 105 own errors.