Deprecation: subprocess based build dependency installation & `PIP_CONSTRAINT` for build dependencies

[ichard26]

The PIP_CONSTRAINT environment variable affecting build dependencies is deprecated and slated for removal in pip 26.1.

In addition, there is an alternative installation flow for build dependencies - inprocess-build-deps which will install build dependencies in the same main pip install/wheel/whatever process. It's currently experimental, gated by inprocess-build-deps, but I'd like to enable it by default in a future pip release. Perhaps pip 26.1 or 26.2? This feature depends on --build-constraint FWIW.

Once both are fully desupported, we can remove a bunch of janky code. These issues will also be resolved:

• pip should ignore [install] no-deps = true in pip.conf when setting up isolated build environment. #11275
• PEP 517 build process fails to take credential from interactive session #10665 (although I need to file another PR so the prompt and spinner don't fight each other)
• no-cache-dir is not respected for deps that require building #12031
• Passing --use-pep517 doesn't apply to build requirements #10739