The change also fixes a rare bug introduced in 9007958:
when there are duplicated keys, insert_or_update overrides
previously inserted ones, emplace doesn't.
fixes: p-ranav/argparse#59
This change also explicitly lists the source files
for CMake. This is because `GLOB` does not remove
old files when switching branches in Git, and
`CONFIGURE_DEPENDS` will add unstaged files after
stashing.
See also: https://cmake.org/cmake/help/latest/command/file.html#glob
