RFR [15] 8238291: Fix inconsistencies in the format of the index files

[Pavel Rappo]

Hello,

Please review the change for https://bugs.openjdk.java.net/browse/JDK-8238291:

  http://cr.openjdk.java.net/~prappo/8238291/webrev.00/

This is a tiny cleanup change. The goal is to fix inconsistency in abbreviating
the fields of the index structure.

The fields of the index structure, such as "label", "containingModule",
"containingPackage", "containingClass", "holder", and "description" are
abbreviated in the index files to a single letter, "l", "m", "p", "c", "h", and
"d" respectively. The "url" field is not. Sometimes it is referred to as "u",
other times as "url".

From my archival research it seems that this peculiarity has been there from day 1. 
While "url" was used for indexing packages, types, members, and then modules,
"u" was used for everything else, which back then comprised the "search tags"
category. I couldn't find any evidence of that being done for some purpose.
I do believe it's just a typo, thus I propose to fix it.

Incidentally, this change reduces the size of the index files. In case you
wonder the reduction is 1% (one percent), otherwise known as "not something to
brag about".

Thanks,
-Pavel