3.0: Deprecate pecl sources.

[henderkes]

We can't remove them entirely just yet, some super old extensions don't support pie yet. But releases on pecl are slow to see release and use worse hosting. Packagists infrastructure is better suited.

[crazywhalecc]

At present, I don't think switching all to PIE is practical. Instead, I implemented pecl type downloads in v3, which allows for very simple configuration of pecl download sources.

However, we can gradually migrate pecl extensions to Pie one by one, should we encounter issues with older versions in the future.