fix theme file collection for duplicate names

[stklcode]

Part of reported regressions in the WP support forums: https://wordpress.org/support/topic/virus-alert-since-1-4-0/

WP_Theme::get_files() returns an associative array with the relative file path as key and the absolute path as value. For child themes with files matching the parent's relative paths, these got overwritten by array_merge(). We only process the absolute path, so now we strip
the values and use numeric indices which are not squashed.

Example:

$child_files = array(
	'functions.php' => '/child/functions.php',
	'header.php'    => '/child/header.php',
);
$parent_files = array(
	'functions.php' => '/parent/functions.php',
	'footer.php'    => '/parent/header.php',
);

$all_files = array_merge( $child_files, $parent_files );
/*
 * array (size=3)
 *   'functions.php' => string '/parent/functions.php' (length=21)
 *   'header.php'    => string '/child/header.php' (length=17)
 *   'footer.php'    => string '/parent/footer.php' (length=18)
 */

$child_files  = array_values( $child_files );
$parent_files = array_values( $parent_files );
$all_files    = array_merge( $child_files, $parent_files );

/*
 * array (size=4)
 *   0 => string '/child/functions.php' (length=20)
 *   1 => string '/child/header.php' (length=17)
 *   2 => string '/parent/functions.php' (length=21)
 *   3 => string '/parent/footer.php' (length=18)
 */

I've added a unit test for the Antivirus_CheckInternals class which does not yet verify any content processing, but it covers the file collection.

[codecov-io]

Codecov Report

Merging #86 (42cdfea) into develop (9530419) will increase coverage by 13.78%.
The diff coverage is 100.00%.

@@              Coverage Diff               @@
##             develop      #86       +/-   ##
==============================================
+ Coverage      26.51%   40.29%   +13.78%     
  Complexity       147      147               
==============================================
  Files              5        5               
  Lines            513      546       +33     
==============================================
+ Hits             136      220       +84     
+ Misses           377      326       -51     

Impacted Files	Coverage Δ	Complexity Δ
inc/class-antivirus-checkinternals.php	47.10% <100.00%> (+47.10%)	38.00 <13.00> (ø)
inc/class-antivirus.php	21.93% <100.00%> (+8.70%)	83.00 <0.00> (ø)

---

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