Add genphdfiles.php: generate PhD auxiliary files from doc-base

[lacatoire]

Following #253 (comment), this moves the generation of the PhD auxiliary files (version.xml, sources.xml, fileModHistory.php) out of doc-base/configure.php into PhD.

genphdfiles.php reads a doc-base/temp/phd-conf.json handoff and writes the three files where the PHP package already consumes them, so configure.php can focus on assembling and validating the XML.

Companion: php/doc-base#318 (removes the generation from configure.php).
Order: merge this first, then the build pipeline calls php phd/genphdfiles.php doc-base/temp/phd-conf.json, then the doc-base PR can land.

Verified byte-identical to current configure.php output on en/fr/de/es/ru. Tests under tests/genphdfiles/.

[alfsb]

Suggestions, for now or the far future.

Where the files are saved in the their historical paths (directly inside doc-base), also save then in the idempotent path, inside doc-base/temp/, prefixed with phd and the rest of name in your preference (the old function names of configure shows a nice symmetry).

After this, each filegen function of genphdfiles.php can guads the existence of files in idempotent path, so they not regenerated each time genphdfiles.php runs, making this step very fast while developing PhD (one run of doc-base/configure, many runs of PhD). doc-base/configure.php always erase doc-base/temp, so any files here never go out of sync with the manual.xml generated here.

This will make all the new files siblings of doc-base/temp/manual.xml. In the far future, if/when PhD is changed to input files only from doc-base/temp, all generated files outside doc-base/temp can stop being duplicated, making both doc-base and PhD fully idempotent.

The idempotent/cached/faster bits are very relevant for phd_sources(), but may not be true or relevant for the other files.

[alfsb]

While the byte-identical result is important for now, this change places these files under full control of PhD, where they belong, so after the move, they can freely be reworked to strictly serve PhD necessities.

Moving the output files to idempotent paths will make their contents fully dependent on doc-base/configure.php runs, as they are now. Other doc-base dependent files can also use doc-base/temp, if there is any.

[lacatoire]

@alfsb I was working on it, while you comment. I already pushed the cache in the latest commit (temp/phd-sources.xml, short-circuit on hit, byte-identical historical output, plus a test). Fully agree on the rest: now that PhD owns these files they can be reworked for PhD's needs, and eventually read straight from temp/ so the historical copies go away. Keeping that out of this PR to land the move cleanly.