Upstream implemented the feature itself in r-lib#493's sibling
r-lib#492, so the conflict resolves to removing this patch.

This branch vectorized `cpp_source()`'s `file` argument:
`all(file.exists(file))` instead of `file.exists(file)`,
`vapply(file, generate_cpp_name, ...)` instead of one name,
and a shared-library name derived from the first file
rather than the last.

r-lib#492 teaches `cpp_source()` to source multiple files
outright, and covers each of those points:
it checks `all(file.exists(file))` and reports the first missing path,
maps `generate_cpp_name()` over `file` with `vcapply()`,
and derives the library name from `generate_package_name()`,
which sidesteps the naming problem this patch worked around.
It also adds fixtures and a test, which this patch never had.

Davis Vaughan closed the upstream pull request carrying this patch,
r-lib#337, with "Closed via r-lib#492 for a similar debugging use case".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K8MneV8KqHYUuC8fWV3X5Q