internal/pkg: create symlinks for files

These are much easier to handle than hard links and should be just as transparent for this use case.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -305,7 +305,7 @@ func TestCache(t *testing.T) {
 			if _, _, err := c.LoadFile(zeroIdent); !reflect.DeepEqual(err, wantErrNonexistentZero) {
 				t.Fatalf("LoadFile: error = %#v, want %#v", err, wantErrNonexistentZero)
 			}
-		}, pkg.MustDecode("G4u4W77C3u46oSAzwPTERKbS9h76iIvcd7Zl8p8Y6hTMb4_QGpH0Glg_DIJg-Usa")},
+		}, pkg.MustDecode("St9rlE-mGZ5gXwiv_hzQ_B8bZP-UUvSNmf4nHUZzCMOumb6hKnheZSe0dmnuc4Q2")},
 
 		{"directory", nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
 			id := pkg.KindTar.Ident(