solver: simplify edge-related functions in the solver

[jsternberg]

Many of the functions in the solver that operated on the edges were very large which obscured their usage and the overall flow of the scheduler. This change refactors those functions into separate smaller functions to make it easier to follow the overall flow of the scheduler.

[tonistiigi]

This is different behvior that captures last return value.

[jsternberg]

This is likely the cause of the test failure then but I did not realize an or operation in Go could short circuit right to left. I figured it was the same as most languages where this was always evaluated left to right.

[tonistiigi]

Ah, I looked over the || first time and just saw the variable setting happened on each iteration. This looks correct to me for return value, but it does exec true = true in a loop if the first item happens to be return true.

[jsternberg]

Figured out what the original issue was. I misread a condition in creating the slow cache request function. I thought it was e.preprocessFunc(dep) != nil && e.slowCacheFunc(dep) != nil but it was an || instead.

I believe this logic is correct. I can change it to be something like:

if e.processUpdate(upt) {
    depChanged = true
}

But I thought the extra brevity may be easier to understand. Not a substantial difference between the two though.