Panics in shipped code that bad input may reach #175

Two panics in non-test code could take input a user controls:

• ProjectContext::find_schema_by_filename (package/config/ir/context.rs:150) panics with "Only disk lookups are supported at the moment" for a package not loaded from disk. Checked: not reachable today. Its only caller freezes a schema_paths key, and schema_paths is no longer a recognised config.idp key: interpreting a virtual package with it returns "'schema_paths' is not a recognised package-manifest key" before the freezing code runs. The branch is dead code.
• from_source (schema/ir/compiler/mod.rs:36) panics on a syntax error. Only a test calls it today, but it's a public entry point.

Fix: return an error from from_source, and remove the dead schema_paths freezing branch (or return an error from it).

Found on core master (a8aee68). The autodoc todo!() stubs are already core#101.