Re: package.el: soft dependencies?

[Artur Malabarba]

If package.el is going to do this, it might be good to add support for
optional dependencies in other places too, like in the byte-compiler.

For instance, we could have a form like the following

(require-optionally 'projectile
  (projectile-ag projectile-dired ...))

When the byte-compiler reads this, it will try to require projectile.
If projectile is available, great, if it isn't then the compiler
treats all those listed functions and macros as `ignore'.

This would sometimes make it more convenient for the developer to
support optional features, because you would only need to account for
the possibility of them return nil, instead of always being forced to
check if they're `fbound'. Though sometimes you'd still need to check
to manually.