Eeschema/CLI: cannot convert unpacked symbol library to packed library

Description

From kicad-cli, it's possible to export a 'traditional' packed symbol library to an unpacked one by adding a trailing slash:

kicad-cli sym upgrade --force -o /tmp/unpacked.kicad_sym/ ~/src/kicad-symbols/4xxx.kicad_sym

However, the reverse is not true - any attempt to use an unpacked library results in an error:

/kicad/kicad-cli sym upgrade -o /tmp/repacked.kicad_sym /tmp/packed.kicad_sym
Symbol library does not exist or is not accessible

This is true regardless of if the packed library is given a trailing slash or not.

This should be possible, as it allows, for example, developing a library in unpacked mode, but packing as a build step and distributing as packed libraries.

KiCad Version

Application: kicad-cli x86_64 on x86_64

Version: 9.99.0-5396-g41367746e2, debug build

Libraries:
	wxWidgets 3.2.8
	FreeType 2.14.1
	HarfBuzz 12.1.0
	FontConfig 2.17.1
	libcurl/8.16.0 OpenSSL/3.6.0 zlib/1.3.1 brotli/1.1.0 zstd/1.5.7 libidn2/2.3.7 libpsl/0.21.5 libssh2/1.11.1 nghttp2/1.68.0 nghttp3/1.12.0

Platform: EndeavourOS, 64 bit, Little endian, wxBase, , x11

Build Info:
	Date: Feb  2 2026 17:50:22
	wxWidgets: 3.2.8 (wchar_t,wx containers) GTK+ 0.0
	Boost: 1.89.0
	OCC: 7.9.1
	Curl: 8.16.0
	ngspice: 44.2
	Compiler: GCC 15.2.1 with C++ ABI 1020

Build settings:
	KICAD_USE_EGL=ON
	KICAD_IPC_API=ON
	KICAD_USE_PCH=OFF
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE_ADDRESS=OFF
	KICAD_SANITIZE_THREADS=OFF

Locale: 
	Lang: en_GB
	Enc: UTF-8
	Num: 1,234.5
	Encoded кΩ丈: D0BACEA9E4B888 (sys), D0BACEA9E4B888 (utf8)