Overhaul pkg-config and CMake lookup, fixing the latter

First, I noticed there was a dangling use of now-removed cross_info in
the CMake lookup. No tests had caught this, but it means that CMake deps
were totally broken. [It also meant that CMake could not be specified
from a native file.]

In a previous of mine PR which removed cross_info, I overhauled finding
pkg-config a bit so that the native and cross paths were shared. I
noticed that the CMake code greatly resembled the pkg-config code, so I
set about fixing it to match.

I then realized I could refactor things further, separating caching,
finding alternatives, and validating them, while also making the
validations less duplicated. So I ended up changing pkg config lookup a
lot too (and CMake again, to keep matching).

Overall, I think I have the proper ideom for tool lookup now, repated in
two places. I think it would make sense next to share this logic between
these two, compilers, static linkers, and any other tool similarly
specifiable. Either the `BinaryTable` class in environment.py, or a new
class for `Compiler` and friends to subclass, would be good candidates
for this.

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