Behavior of `strip_prefix.files_only()` and `strip_prefix.from_pkg()` swapped between `pkg_files` and `pkg_tar`

There is a bug that is confusing most users of `pkg_tar` (like it did with me): the effects of using `strip_prefix.files_only()` and `strip_prefix.from_pkg()` work as advertised when using it with `pkg_files`, but the behavior is unexpectedly swapped when using it with `pkg_tar`.

Below is a full repro:

```bash
#!/bin/bash

set -eu

echo '6.4.0' > .bazelversion

mkdir subdir
touch one.txt subdir/two.txt

cat >WORKSPACE <<'EOF'
load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
http_archive(
name = "rules_pkg",
urls = [
"https://mirror.bazel.build/github.com/bazelbuild/rules_pkg/releases/download/0.9.1/rules_pkg-0.9.1.tar.gz",
"https://github.com/bazelbuild/rules_pkg/releases/download/0.9.1/rules_pkg-0.9.1.tar.gz",
],
sha256 = "8f9ee2dc10c1ae514ee599a8b42ed99fa262b757058f65ad3c384289ff70c4b8",
)
load("@rules_pkg//:deps.bzl", "rules_pkg_dependencies")
rules_pkg_dependencies()
EOF

cat >BUILD.bazel <<'EOF'
load("@rules_pkg//pkg:mappings.bzl", "pkg_files", "strip_prefix")
load("@rules_pkg//pkg:tar.bzl", "pkg_tar")

# Expected behavior

# `strip_prefix.from_pkg()` in `pkg_file` dependency produces expected non-flat layout:
# one.txt
# subdir/two.txt
pkg_files(
name = "pkg-files-strip-from-pkg",
srcs = [
"one.txt",
"subdir/two.txt",
],
strip_prefix = strip_prefix.from_pkg(),
)
pkg_tar(
name = "pkg-tar-nonflat-ok",
srcs = [":pkg-files-strip-from-pkg"],
)

# `strip_prefix.files_only()` in `pkg_file` dependency produces expected flat layout:
# one.txt
# two.txt
pkg_files(
name = "pkg-files-strip-files-only",
srcs = [
"one.txt",
"subdir/two.txt",
],
strip_prefix = strip_prefix.files_only(),
)
pkg_tar(
name = "pkg-tar-flat-ok",
srcs = [":pkg-files-strip-files-only"],
)

# Unexpected behaviors

# `strip_prefix.files_only()` in `pkg_tar` with inlined files in `srcs` produces unexpected non-flat layout:
# one.txt
# subdir/two.txt
pkg_tar(
name = "pkg-tar-flat-unexpected",
srcs = [
"one.txt",
"subdir/two.txt",
],
strip_prefix = strip_prefix.from_pkg(),
)

# `strip_prefix.from_pkg()` in `pkg_tar` with inlined files in `srcs` produces unexpected flat layout:
# one.txt
# two.txt
pkg_tar(
name = "pkg-tar-nonflat-unexpected",
srcs = [
"one.txt",
"subdir/two.txt",
],
strip_prefix = strip_prefix.files_only(),
)
EOF

for target in pkg-tar-nonflat-ok pkg-tar-flat-ok pkg-tar-flat-unexpected pkg-tar-nonflat-unexpected; do
echo "* Building target: $target"
bazel build "//:$target"
echo "Contents:"
tar tf "bazel-bin/$target.tar"
echo
done
```

(Side note: in both `pkg_tar` and `pkg_files`, the default behavior is to flatten the files when `strip_prefix` is unset, which is consistent)