fix: Do not load the resilient base class for feature detection

[Omikhleia]

It loaded the whole of the extensions provided by silex.sile, which is not what we wanted outside the resilient usage. It behaves badly with silex.sile 0.4 as overrides would now come too late possibly, causing havoc...

A quick workaround for #107 which would deserve a cleaner solution...

[Omikhleia]

Test ✔️

$ sile -c book -u packages.markdown -u inputters.djot temp/xx.dj -d markdown -d silex
SILE v0.14.16 (LuaJIT 2.1.1702233742)

[silex] Providing SU.ast compatibility (SILE 0.15)
[silex] Patching language support for BCP47
[markdown] Used in a non-resilient class
[silex] Loaded language feature support for en-GB: matched en
[silex] Load localized strings for en-GB: matched en
[silex] Loaded language feature support for en: matched en
[silex] Load localized strings for en: matched en

Vs.

$ sile -c resilient.book -u packages.markdown -u inputters.djot temp/xx.dj -d markdown -d silex
SILE v0.14.16 (LuaJIT 2.1.1702233742)

[silex] Providing SU.ast compatibility (SILE 0.15)
[silex] Overriding require global function
[silex] Re-loading outputter libtexpdf
[silex] Loaded silex version of outputters.base
[silex] Loaded silex version of outputters.libtexpdf
[silex] Re-instanciating current outputter libtexpdf
[silex] Loading extra inputters if available
[silex] Patching core for multiple instantiation side effects
[silex] No need for patching utilities (AST position stripping)
[silex] No need for patching pre-0.14.9 issues
[silex] Patching language support for BCP47
[silex] Loaded silex version of classes.base
[silex] Loaded language feature support for und: matched und
[silex] Load localized strings for und: matched und
[silex] Loaded silex version of typesetters.base
[silex] Loaded silex version of packages.scalebox
[silex] Ignoring package already loaded in the class: inputfilter
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: counters
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: raiselower
[silex] Ignoring package already loaded in the class: rebox
[silex] Ignoring package already loaded in the class: counters
[silex] Ignoring package already loaded in the class: infonode
[silex] Ignoring package already loaded in the class: rebox
[markdown] Used in a resilient class
[silex] Ignoring package already loaded in the class: image
[silex] Ignoring package already loaded in the class: inputfilter
[silex] Ignoring package already loaded in the class: labelrefs
[silex] Ignoring package already loaded in the class: rebox
[silex] Ignoring package already loaded in the class: struts
[silex] Ignoring package already loaded in the class: rules
[silex] Ignoring package already loaded in the class: textsubsuper
[silex] Loaded silex version of packages.url
[silex] Ignoring package already loaded in the class: inputfilter
[silex] Loaded silex version of packages.pdf
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: counters
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: svg
[silex] Ignoring package already loaded in the class: resilient.styles
[silex] Ignoring package already loaded in the class: raiselower
[silex] Ignoring package already loaded in the class: rules
[silex] Ignoring package already loaded in the class: markdown.commands
[silex] Ignoring package already loaded in the class: markdown.commands
[silex] Loaded language feature support for en-GB: matched en
[silex] Load localized strings for en-GB: matched enFontconfig error: No writable cache directories
[silex] Loaded language feature support for en: matched en
[silex] Load localized strings for en: matched en