Report warning when built-in package gets shadowed

[raxod502]

See #1146 (comment).

There might be a more comprehensive solution to avoid this situation of multiple package versions happening in the first place, but it is a simple improvement for everyone if we report a warning when we detect that the situation has happened.

[tomrss]

@raxod502 in addition to the warning, I suggest adding the possibility to exclude pulling in certain package dependencies.
Something like this:

(straight-use-package 'eglot-java
   :exclude-dependencies (eglot jsonrpc))

In my opinion, this can be very useful to:

• avoid pulling unnecessary packages that are already built-in
• resolve nasty dependency conflicts, in more general cases than only built-in packages
• protect against packages with ill defined dependencies

Also, this feature is implemented in notable package managers like npm, maven, apt, pip (not real package manager, but still) and also guix, that is like straight a functional package manager.

Honestly I'm struggling with this from a while, I had to drop every package that depend on built-in packages with Emacs 30.

[raxod502]

As a workaround, you should be able to stub out any package by registering or overriding it with a :type built-in recipe

[tomrss]

Very useful, thanks for the advice!