bp_core_is_root_component() missing in BP 1.5+

[DJPaul]

The bp_core_is_root_component function did not make it into the 1.5 branch or trunk. It was removed in r3728/#2600.

I found this when updating a site from 1.2.x to 1.5.x which was using this in a custom plugin.

[djpaul]

(In [6474]) Bring back bp_core_is_root_component(), and deprecate it in favour of bp_is_root_component().

• This function was renamed in r3728/#2600 for 1.5, but we broke backpat as we missed adding the deprecated version.

Fixes #4241

[djpaul]

(In [6475]) Bring back bp_core_is_root_component(), and deprecate it in favour of bp_is_root_component() (1.6 branch version).

• This function was renamed in r3728/#2600 for 1.5, but we broke backpat as we missed adding the deprecated version.

Fixes #4241