bash-completion: complete file aliases

[shbernal]

Summary:

• resolve file-valued aliases in Bash completion before completing option values
• let list-operation aliases such as --sub-file complete filesystem paths via their parent file option
• reuse the same path for related aliases such as --audio-file, --external-file, --cover-art-file, and --glsl-shader

Fixes #7955

Tests:

• bash -n etc/mpv.bash-completion
• manual tiny Bash completion check for --sub-file returning subtitle.srt