Determine autoremovable kernels at run-time

Our kernel autoremoval helper script protects the currently booted
kernel, but it only runs whenever we install or remove a kernel,
causing it to protect the kernel that was booted at that point in time,
which is not necessarily the same kernel as the one that is running
right now.

Reimplement the logic in C++ such that we can calculate it at run-time:
Provide a function to produce a regular expression that matches all
kernels that need protecting, and by changing the default root set
function in the DepCache to make use of that expression.

Note that the code groups the kernels by versions as before, and then
marks all kernel packages with the same version.

This optimized version inserts a virtual package $kernel into the cache
when building it to avoid having to iterate over all packages in the
cache to find the installed ones, significantly improving performance at
a minor cost when building the cache.

LP: #1615381

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