Pack targets should be preferred over builtin targets when targets of the same name exist in both

Right now, if there's a builtin target that matches the requested target, it is used over any pack targets. This is undesirable as pack targets may provide more functionality than builtin targets (such as having peripherals).

Specifically, I'm concerned with RP2040 target. There's a vendor published pack with contains peripheral SVDs for all the peripherals, which is very convenient when debugging and looking and peripheral registers, e.g. with 'monitor rr ...'.

Both the pack and the builtin registers targets under the name rp2040.

I think pyocd should, when a pack with the same target name as a builtin target is installed, prefer the pack target. The code right now only attempts to load target from a pack if there's no existing target in the list (which initially contains builtin targets).

I'm willing to make the necessary changes if we can nail down the behavior we want to have.

Start by tracing the target-loading logic described in the issue, focusing on how builtin and pack targets are collected and resolved when both provide rp2040. Confirm the desired precedence for an installed pack target, then verify that the pack target is selected while existing builtin behavior remains intact.