fix: keep an existing package.json when --no-pack is passed

[cpruijsen]

Fixes #1521.

create_pkg_dir in src/command/utils.rs opens by deleting package.json from the out dir:

let _ = fs::remove_file(out_dir.join("package.json")); // Clean up package.json from previous runs

That is right when wasm-pack is about to write a new one, and wrong under --no-pack, where it
never will. The user's own file is removed and nothing replaces it. --no-pack reads as "do not
generate one", not "delete the one I put there", which is what the issue reports.

The flag is threaded through and the removal is skipped when it is set. create_pkg_dir already
takes no_gitignore for the same reason, so no_pack sits beside it rather than introducing a new
shape:

pub fn create_pkg_dir(out_dir: &Path, no_gitignore: bool, no_pack: bool) -> Result<()>

Worth flagging since the function is pub: that is a signature change for anything outside this
repo calling it. The two callers inside it are updated, and the fourteen call sites in
tests/all/manifest.rs take false.

Only the removal is gated. The directory is still created and .gitignore still written under
--no-pack, both unchanged.

Test

it_should_not_delete_existing_pkg_json_if_passed_no_pack writes { "name": "pre-existing" } into
pkg/package.json, builds with --no-pack, and asserts the file still reads back byte for byte.

It pins the change rather than merely passing: with the guard reverted it fails on the read, and its
sibling it_should_not_make_a_pkg_json_if_passed_no_pack still passes, so the two cover the two
halves of what --no-pack means.

cargo check --all-targets and cargo fmt --check are clean.

[cpruijsen]

Closing this one so it does not sit in the queue.