Fetch GitHub SPDX license metadata during plugin updates and emit meta.license for generated vim plugin derivations.
Fall back to lib.licenses.unfree when no SPDX license is detected, and round-trip existing license metadata through get-plugins.nix so partial updates do not drop it.
Keep fetcher tag metadata when reading current plugin state from Nix.
The updater needs to know whether a plugin was fetched by tag or by commit.
Without carrying src.tag through, tagged sources are read back as if rev were
always the canonical source identifier, which breaks tag-aware comparisons and
cache behavior later in the update flow.
buildNeovimPlugin removed in previous update, but get-plugins is trying
to call package with removed argument.
Signed-off-by: Austin Horstman <khaneliman12@gmail.com>
