miner: Misc small refactor and renaming

Summary:
miner: Remove mentions of 'package' in names

We no longer have the concept of packages in the block generation code.
This commit renames the following:
- `TestPackage` to `TestTx`, plus rename its args
- `addPackageFrac` to `addTxsFrac`
- `addPackageTxs` to `addTxs`

Also updated comments.

Note that TestTx has been renamed to TestTxFits. We already have a poorly named CheckTx method here, no need for another layer of confusion.

miner: Add fPrintPriority member variable

There is no reason to keep checking this arg each time we add a tx. Best
to cache it once at class construction. This saves cycles.

Port of bchn#1118.

Depends on D13121.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, sdulfari

Reviewed By: #bitcoin_abc, PiRK, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13122