Rewrite lib/pecl

[Frzk]

This is a complete rewrite of lib/pecl which originally contains code to install any PECL extension.

While trying to reproduce #236, I dove into this code and thought it would greatly benefit from a rewrite. So, this PR:

• Splits the code into multiple, small and understandable functions, making it easier to debug/maintain/extend,
• Introduces some kind of namespace to better identify the functions and where they come from,
• Ditches the set +e that was put at some point,
• Should fail early when compiling an extension fails, unlike currently,
• Should fail early when trying to install a non-existing extension, unlike currently,
• Has better error and success messages,
• Includes built-in support for some extensions (oci8, yaml and scoutapm). This part is maybe the most arguable (see the php::pecl::enable_pecl_extension and php::pecl::configure_extension functions).

Fixes #236

[Soulou]

The code looks great, it's tricky to validate if it works correctly because of the lack of tests.

What do you think about using this PR to introduce some testing in the buildpack to prevent regressions?

Or maybe merge it and then prepare another PR with the ability to test the behavior?

What do you think would be best?

[Frzk]

I'd be keen to:

1. write the tests,
2. make them pass with the current code,
3. make them pass with the new code (from the PR).

WDYT?

[Soulou]

@Frzk Look good to me, but do you prefer this PR being merged or wait later?