…nalysis phase performance (#3381)

When py_binary/py_test were being built, they were flattening the
runfiles
depsets at analysis time in order to create the zip file mapping
manifest for
their implicit zipapp outputs. This flattening was necessary because
they had
to filter out the original main executable from the runfiles that didn't
belong
in the zipapp. This flattening is expensive for large builds, in some
cases
adding over 400 seconds of time and significant memory overhead.

To fix, have the zip file manifest use the `runfiles_with_exe` object,
which is
the runfiles, but pre-filtered for the files zip building doesn't want.
This
then allows passing the depsets directly to `Args.add_all` and using
map_each
to transform them.

Additionally, pass `runfiles.empty_filenames` using a lambda. Accessing
that
attribute implicitly flattens the runfiles.

Finally, because the original profiles indicated `str.format()` was a
non-trivial
amount of time (46 seconds / 15% of build time), switch to using `+`
instead.

This is a more incremental alternative to #3380 which achieves _most_ of
the
same optimization with only Starlark changes, as opposed to introducing
an
external script written in C++.

[Profile of a large
build](https://github.com/user-attachments/assets/e90ae699-a04d-44df-b53c-1156aa890af5),
which shows a Starlark CPU profile. It shows an overall build
time of 305 seconds. 46 seconds (15%) are spent in `map_zip_runfiles`,
half of which
is in `str.startswith()` and the other half in `str.format()`.

---------

Co-authored-by: Richard Levasseur <rlevasseur@google.com>