cmd/compile/internal/importer: flip enable alias to true [1.23 backport]

[xieyuschen]

This issue is created to request a backport at go1.23 to fix #70394. The issue doesn't exist in go1.24(master) as the code has refactored a bit before and fixed this issue.

go/src/cmd/compile/internal/importer/ureader.go

Line 34 in b0bbfb1

enableAlias: true,

The backport fix allows ureader to read exportdata with alias correctly and it doesn't affect the correctness of compile to construct exportdata for the following packages.

@timothy-king

[gopherbot]

Change https://go.dev/cl/629997 mentions this issue: [release-branch.go1.23] cmd/compile: enable alias for pkgReader