CRM-20849: Allow duplicate PSR-4 prefixes

[mickadoo]

Overview

Currently when adding prefixes from an extension info.xml it uses the method setPsr4 which replaces and existing PSR-4 mapping with the same prefix.

We should allow multiple extensions to use the same namespace, particularly since the Civi\ namespace (which is used by core) is likely to be copied by extension authors.

Before

Two extensions using the same PSR-4 prefix will only allow classes from one to be autoloaded.

After

Multiple extensions can use the same PSR-4 prefix.

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• CRM-20849: Multiple extensions using the same autoloader prefix will overwrite previous

[colemanw]

@totten can you please review this.

[colemanw]

I just encountered a fatal error on a site with both api4 and civicase extensions running (unable to load a class in the \Civi namespace). Applying this patch fixed it :)

[totten]

Thanks. Nicely reported.

• From a design perspective, looks good.
• From a testing perspective, I would'a basically done what Coleman just reported, so that seems good.