Search api should not set the info for search server modules as `required`

[argiepiano]

This is a similar issue as the one discovered in Feeds. The problem here is that search_api checks if there is a search server currently in use by indexes, and if it finds any, then it marks the server module's info as required. In Backdrop, this has the effect of completely hiding the module, rather than disabling the checkbox to disable it. This produces WTF moments, as it's impossible to tell what database_search, for example, is enabled but not shown.

[argiepiano]

In Backdrop, the info should be switched to disabled - which disables the checkbox, but still shows the module.

[argiepiano]

@laryn, PR #61 is ready for review.

[laryn]

Thanks @argiepiano!