758: improve performance

[asgrim]

Fixes #758

supersedes #759 ; although that PR solved the immediate issue, the root cause was really careless management of the repository; there were multiple instantiations of the platform repo, which cascaded into so many PhpBinaryPath calls. Some calls have been eliminated, or run with -n to speed up slightly; but primarily we've removed the need to call those so many times anyway.

On OSX I was originally seeing:

Processes started: 925 in 18.52s
  php: 114 in 14.57s (127.8ms avg)
  pkg-config: 810 in 3.94s (4.9ms avg)
  php-config: 1 in 0.01s (7.2ms avg)

Now after this change, seeing:

Processes started: 22 in 1.30s
  php: 15 in 1.24s (82.6ms avg)
  pkg-config: 6 in 0.06s (9.6ms avg)
  php-config: 1 in 0.01s (6.6ms avg)

(this was tested with command

time PIE_DEBUG_PROCESS_COUNT=1 CPATH=/opt/homebrew/include bin/pie install mongodb/mongodb-extension apcu/apcu pecl/pcov phpredis/phpredis noisebynorthwest/php-spx xdebug/xdebug bsn4/grpc  --with-zlib-dir=/opt/homebrew/opt/zlib -v

This should serve as a good improvement to performance on OSX platforms, where this performance issue seemed to be most prevalent. It's improved Linux performance too, but it will be less noticeable anyway as it was already fast.

PR submitter checklist

• I have read CONTRIBUTING.md
• I discussed this feature with the maintainers in Slow no-op installs on macOS: repeated PHP and pkg-config subprocesses #758
• I have added appropriate tests
• I confirm that I have the right to submit this under the project's open source licence