Installing resources directly under ${prefix}/share causes conflicts
with other packages. This will become more problematic whenever Ghostty
is opened and becomes packaged in distributions.
Instead, install all resources under a "ghostty" subdirectory (i.e.
${prefix}/share/ghostty). This includes themes, shell integration, and
terminfo files.
Only "/usr/share" style paths use the "ghostty" subdirectory. On macOS,
Ghostty is already isolated within its app bundle, and if
$GHOSTTY_RESOURCES_DIR is set then we assume that points to the actual
resources dir (without needing to append "ghostty" to it).
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