Remove copying of libcxx headers

Previously, our modulemap generation would generate a modulemap
containing lines such as "header iostream" and "textual header
__assertion_handler".

This meant that we required the generated modulemap to be in the same
directory as all the headers.

generate_system_modulemap.py solves that by rebasing the paths, giving
us "../../third_party/libc++/src/include/iostream" and
"../../buildtools/third_party/__assertion_handler" respectively.

It already does this, but just needs to be reconfigured to support not
combining it with the builtin and libc++ modulemap.

Compile-Size: Size increase is mostly a non-issue and an acceptable tradeoff, since it's just one file required for every compile action and it's cached.
Bug: 508053783
Link: https://chromium-review.googlesource.com/id/If57de6c8a6941c0b89366b9966ed1a416a6a6964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7809476
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1628367}
NOKEYCHECK=True
GitOrigin-RevId: b8376384c8220d01a556dee1f9e91bd34b68cb9b