API Remove configuration of _resources path

[GuySartorelli]

NOTE This will need to be targetted at 2 once that branch has been created (see silverstripe/silverstripe-framework#10349)

BC breaking changes

• Removed Library::DEFAULT_RESOURCES_DIR
• Set Library::RESOURCES_PATH to explicitly be "_resources"
• Removed Library::getResourcesDir()

Parent issue

• Default resources folder to _resources silverstripe-framework#10399

[michalkleiner]

Why would we remove the configurability of this and not just change the default?

[GuySartorelli]

That's what the issue is for, but I realised that after doing the work. The configuration feels a bit like tech debt to me but have asked for clarification of the intent.

[michalkleiner]

I reckon once it was made configurable it feels wrong to force users into something opinionated again, so I would vote for changing the default to _resources so it's consistent with what some other modules had configured, but leave it configurable so that devs can still leverage that in project code if they wanted to. I imagine not everyone is using _resources, might be using something custom and have other systems/URLs etc depending on it.

[GuySartorelli]

Closed in favor of #57