completion: bash: properly recurse targets

[casperdcl]

• properly recurse commands & subcommands path completion
  • only autocomplete DVC-files where appropriate
• add inline documentation for maintenance
• fixes completion: bash: handle subdirs #3361
• fully fixes Autocomplete for file names doesn't work (partially) [bash] #1923
• continues Fix bash completion #3360
• related tab completion scripts need more love #2985

Notes

• -o nospace is required (as documented in the code) to avoid an unreasonable rewrite of everything.
• the snap package automatically benefits from this PR :)
• I'm tempted to write a test for future use, and perhaps even think of a way to automate based on argparse but that way likely lieth a massive independent project moste arcane.
  • have manually tested all current permutations though

I haven't seen any code examples take this approach and don't know why. It looks elegant, maintainable, and efficient to me.

[casperdcl]

I haven't seen any code examples take this approach and don't know why. It looks elegant, maintainable, and efficient to me.

[efiop]

Neat!