php.mkExtension: make source name agnostic

PHP obtained from Git contains files directly in the top-level directory,
which will be placed in Nix store. The generic builder will then copy it
to the /build sandbox as its basename with hash stripped (e.g. source/).
This breaks the assumption of `mkExtension`, which expects that PHP’s
source will be extracted into `php-«version»/` directory, since that
is what the release tarballs contain.

Let’s unset the `sourceRoot`, leaving it to `unpackPhase` to find
the source directory. Then, we will be able go to the extension
directory from there.

We use custom pre-configure phase so that both `postPatch` and
`preConfigure` is available to consumers.

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