build: remove error return from genMainModule

The genMainModule function never returns an error, so simplified
its signature to return only Package.

Updated:
- genMainModule signature: (Package, error) -> Package
- Call site in build.go to not handle error
- Both test cases to remove error checking

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <8459+cpunion@users.noreply.github.com>

internal/build/build.go

@@ -782,10 +782,7 @@ func linkMainPkg(ctx *context, pkg *packages.Package, pkgs []*aPackage, outputPa
 		}
 	})
 	// Generate main module file (needed for global variables even in library modes)
-	entryPkg, err := genMainModule(ctx, llssa.PkgRuntime, pkg, needRuntime, needPyInit)
-	if err != nil {
-		return err
-	}
+	entryPkg := genMainModule(ctx, llssa.PkgRuntime, pkg, needRuntime, needPyInit)
 	entryObjFile, err := exportObject(ctx, entryPkg.PkgPath, entryPkg.ExportFile, []byte(entryPkg.LPkg.String()))
 	if err != nil {
 		return err