Track files referenced by `--file`

[viperML]

use_flake, which uses nix print-dev-env accepts an installable, which can be:

• flake
• -f file.nix
• --expr "myexpr"
• /nix/store/...

Additionally, if using a -f file.nix, it won't detect changes on the file automatically.

Proposal

• Rename use flake to use installable
• Properly track changes when using --file

[Mic92]

Sorry, but that's too late. Breaking this interface would cause too much churn throughout the ecosystem and we also want to be compatible direnv's own implementation, which might come from some old debian/centos installation. In the majority of normal use cases it will be a flake so the name is not too bad. People also are more likely to know what a flake is compared to what a installable means.

[Mic92]

We can actually support --file tracking though.