Indexer package

[Omikhleia]

We need a resilient.indexer package for index entries.

(Opened here so as to close a checkbox in Omikhleia/omikhleia-sile-packages#13 since it won't happen there)

[Omikhleia]

Shifting milestones - not doable in the remaining time frame for 2.1
This is kind of a drawback, but heh...

[Omikhleia]

Shifting again - I need 2.2 by November and won't be able to consider this.

[davidchisnall]

In the interim, it would be nice if the resilient.book class could call buildIndex in its end-page routine if it exists, so that the SILE indexer package can be used with it.

[Omikhleia]

In the interim, it would be nice if the resilient.book class could call buildIndex in its end-page routine if it exists, so that the SILE indexer package can be used with it.

That's probably not the correct solution: that way of doing things (that is, calling package methods from the class) was deprecated in SILE.
Packages that need something to occur at end-page should now register to the "endpage" hook.

SILE's indexer package would probably need some love (sile-typesetter/sile#1339) and the fact that it still relies on old 0.12.x solutions should also be addressed.

(Eventually, resilient nevertheless needs its own implementation, style-enabled.)