Fix Pkg.build when run on a non-installed package (#16584)

* Fix Pkg.build when run on a non-installed package

* Avoid SIGTERM when PkgError occurs with Pkg.build

base/pkg/entry.jl

@@ -523,9 +523,9 @@ end
 function build!(pkgs::Vector, buildstream::IO, seen::Set)
     for pkg in pkgs
         pkg == "julia" && continue
-        pkg in seen && continue
-        build!(Read.requires_list(pkg),buildstream,push!(seen,pkg))
+        pkg in seen ? continue : push!(seen,pkg)
         Read.isinstalled(pkg) || throw(PkgError("$pkg is not an installed package"))
+        build!(Read.requires_list(pkg),buildstream,seen)
         path = abspath(pkg,"deps","build.jl")
         isfile(path) || continue
         println(buildstream, path) # send to build process for evalfile
@@ -582,8 +582,8 @@ function build!(pkgs::Vector, errs::Dict, seen::Set=Set())
             end
         end
     catch err
-        kill(pobj)
         close(io)
+        isa(err, PkgError) ? wait(pobj) : kill(pobj)
         rethrow(err)
     finally
         isfile(errfile) && Base.rm(errfile)

test/pkg.jl

@@ -281,4 +281,13 @@ temp_pkg_dir() do
         @test contains(msg, "Pkg.build(\"BuildFail\")")
         @test contains(msg, "Throw build error")
     end
+
+    # issue #15948
+    let package = "Example"
+        Pkg.rm(package)  # Remove package if installed
+        @test Pkg.installed(package) == nothing  # Registered with METADATA but not installed
+        msg = readstring(ignorestatus(`$(Base.julia_cmd()) -f -e "redirect_stderr(STDOUT); Pkg.build(\"$package\")"`))
+        @test contains(msg, "$package is not an installed package")
+        @test !contains(msg, "signal (15)")
+    end
 end