go/ssa: migrate source_test.go away from loader

Also changes buildPackage to return a *packages.Package.
buildPackage no longer builds dependencies.

For golang/go#69556

Change-Id: Id10ccee131394cda657213124907949a0b82f903
Reviewed-on: https://go-review.googlesource.com/c/tools/+/614116
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: timothy-king

go/ssa/builder_generic_test.go

@@ -519,11 +519,11 @@ func TestGenericBodies(t *testing.T) {
 		pkgname := parsePackageClause(t, content)
 		t.Run(pkgname, func(t *testing.T) {
 			t.Parallel()
-			ssapkg, f := buildPackage(t, content, ssa.SanityCheckFunctions)
+			ssapkg, ppkg := buildPackage(t, content, ssa.SanityCheckFunctions)
 			fset := ssapkg.Prog.Fset
 
 			// Collect all notes in f, i.e. comments starting with "//@ types".
-			notes, err := expect.ExtractGo(fset, f)
+			notes, err := expect.ExtractGo(fset, ppkg.Syntax[0])
 			if err != nil {
 				t.Errorf("expect.ExtractGo: %v", err)
 			}