Add coverage support, improve logs and fix bugs - #37

When a PHP package install bundles its own default build of a versioned
extension (e.g. redis ships with its own recent version pre-enabled),
add_pecl_extension correctly detects the version mismatch against the
pinned version and falls back to pecl_install - but it does this even
when a matching build has already been restored from a CI cache into a
separate directory, since shared_extension only checks that *some* file
exists at ext_dir, not which version it is.

This adds an opt-in CACHED_EXTENSIONS_DIR: when set and a mismatch is
detected, look for extension.so there, and if found, copy it into
ext_dir and re-check before falling back to a full pecl_install. When
CACHED_EXTENSIONS_DIR is unset, behavior is unchanged.

Related to shivammathur/cache-extensions#21 and shivammathur#37, where a cached
build of a versioned PECL extension is silently ignored and
recompiled on every run regardless of caching.

…1108)

* fix: honor a cache-restored build when a versioned extension mismatches

When a PHP package install bundles its own default build of a versioned
extension (e.g. redis ships with its own recent version pre-enabled),
add_pecl_extension correctly detects the version mismatch against the
pinned version and falls back to pecl_install - but it does this even
when a matching build has already been restored from a CI cache into a
separate directory, since shared_extension only checks that *some* file
exists at ext_dir, not which version it is.

This adds an opt-in CACHED_EXTENSIONS_DIR: when set and a mismatch is
detected, look for extension.so there, and if found, copy it into
ext_dir and re-check before falling back to a full pecl_install. When
CACHED_EXTENSIONS_DIR is unset, behavior is unchanged.

Related to shivammathur/cache-extensions#21 and #37, where a cached
build of a versioned PECL extension is silently ignored and
recompiled on every run regardless of caching.

* fix: preserve versioned builds in ext_dir instead of a separate cache dir

Per @shivammathur's review on the PR: drop the opt-in
CACHED_EXTENSIONS_DIR env var. Instead, after installing a versioned
extension, keep a second copy of the built .so in ext_dir itself under
a name with no .so suffix (e.g. redis-5.3.7), so it can't be mistaken
for the active extension and rides along in the same ext_dir that
cache-extensions already caches wholesale - no separate cache
directory, key, or workflow step required. On a version mismatch,
restore that preserved build over the active binary and re-check
before falling back to a full pecl_install.