pcre: move to pcre2
pcre: move to pcre2

Summary:
The pcre API is no longer maintained. According to pcre.org:

> The older, but still widely deployed PCRE library, originally released in
> 1997, is at version 8.45. This version of PCRE is now at end of life, and is no
> longer being actively maintained. Version 8.45 is expected to be the final
> release of the older PCRE library, and new projects should use PCRE2 instead.

Let's thus migrate to pcre2.

Reviewed By: chadaustin

Differential Revision: D38134776

fbshipit-source-id: 0d2eaeb84013cd03a7426fffa23afe9f48d57cdf

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