sdl2 external gentoo pkgs (#1557)

* fix: improper syntax that will be rejected post-2.1 (#1417) (#1445)

We had a missing check in index loading logic that should have rejected an unexpected array.

* Add gentoo package names.

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Co-authored-by: Alejandro R Mosteo <amosteo@unizar.es>

index/li/libsdl2/libsdl2-external.toml

@@ -12,4 +12,5 @@ kind = "system"
     'centos|fedora' = ["SDL2-devel"]
     'homebrew' = ["sdl2"]
     'macports' = ["libsdl2"]
+    'gentoo' = ["media-libs/libsdl2"]
     'msys2' = ["mingw-w64-x86_64-SDL2"]

index/li/libsdl2_image/libsdl2_image-external.toml

@@ -13,4 +13,5 @@ kind = "system"
     'fedora' = ["SDL2_image-devel"]
     'homebrew' = ["sdl2_image"]
     'macports' = ["libsdl2_image"]
+    'gentoo' = ["media-libs/sdl2-image"]
     'msys2' = ["mingw-w64-x86_64-SDL2_image"]

index/li/libsdl2_mixer/libsdl2_mixer-external.toml

@@ -13,4 +13,5 @@ kind = "system"
     'fedora' = ["SDL2_mixer-devel"]
     'homebrew' = ["sdl2_mixer"]
     'macports' = ["libsdl2_mixer"]
+    'gentoo' = ["media-libs/sdl2-mixer"]
     'msys2' = ["mingw-w64-x86_64-SDL2_mixer"]

index/li/libsdl2_ttf/libsdl2_ttf-external.toml

@@ -13,4 +13,5 @@ kind = "system"
     'fedora' = ["SDL2_ttf-devel"]
     'homebrew' = ["sdl2_ttf"]
     'macports' = ["libsdl2_ttf"]
+    'gentoo' = ["media-libs/sdl2-ttf"]
     'msys2' = ["mingw-w64-x86_64-SDL2_ttf"]