Use soong's finder to find included makefiles in mk2rbc

Having soong generate a list of makefiles for mk2rbc
to look through is much faster than having mk2rbc search
itself. Profiling the readLinesFromeFile() function that
reads the list of makefiles shows it takes on the order
of 200 microseconds, much faster than the ~5 seconds
it takes for mk2rbc to search the tree itself.

This CL also allows include statements that are prefixed
with a variable. The concern with this was that there
would be a lot of load statemnts emitted for generic
include statements, causing the generated code to look
ugly, and converting and loading all those files could
cause performance issues. On the performance issues
front, there's already a check that it doesn't result
in over 150 potentially included files. We can lower
that number if necessary, but it's probably good for now.
On the generated code front, while it's true that it's
ugly, it's better to have working but ugly generated
code than refusing to generate anything working at all.

To ensure the soong finder step isn't slowed down due
to having to find a bunch of new makefiles, I profiled
the combination of newSourceFinder and FindSources in main.go:

Baseline incremental:
338.011634ms
340.853335ms
348.541762ms
333.229644ms
349.124824ms

Baseline clean:
1.003836419s
1.006203912s
996.193648ms
1.031005604s
1.03691152s

Modified incremental:
349.029285ms
349.264496ms
351.774948ms
337.63187ms
359.425306ms

Modified clean:
1.028238704s
1.053103506s
1.032757506s
1.016631201s
1.04288108s

So we can see the times are barely affected by this change.

Fixes: 213508006
Test: go test
Change-Id: Iab18bfb127ba3b7e63f2c01f69064805a8398764