This provides a convenient way of letting the debugging func inject a
newline after each write (for readability in debugging).

This separates a bunch of the static state/rules/information that comes
from the root project and input parameters into a discrete subsystem.
The only real benefit here is focusing the state tracked by the solver
in on the actual algorithm of solving, and less so these static rules -
which should make it a bit easier for other people to grok.

All changes are geared towards making "default"-type values explicit, as
that increases the likelihood that equivalent inputs will produce
identical hash digests.

Hashing functions are exquisitely sensitive to inputs - that's why
they're useful. But it makes them a PITA to work with. Having an
easy-to-scan visualization of hashing inputs in tests frees up cognitive
capacity to focus on the algorithm.

To further improve debugging of issues with the input hashing, this adds
"section headers" - strings that are output prior to each type of data
that's present in the cache.

Also partially switched to progressive mutation table-based tests for
input hashing, and added test cases that cover salient combinations of
overrides, imports, and constraints.

Makes it easier to see problem spots on a quick scan.

These solve the problem, at least in the hasher, of the possibility for
strings representing different types of versions to collide. For
example, prior to this change, a branch constraint named "foo" and a
version constraint named "foo" could cause the hasher to produce the
same hash, even though the two inputs would not have admitted the same
solution set.