Relay compiler panics if the schema file path starts with `./` and watchman is being used

It seems Watchman does not interpret paths with `./` in the same what that our checks for the existence of the file does. The result is that our initial validation ensuring we have a schema file passes, but when we initialize the project, no schema file is discovered by Watchman and thus when the compiler goes to look up the schema it panics.

I haven't been able to locate exactly the specification for what type of path Watchman expects, but I think we need to either find a way to correctly transform arbitrary paths into that format, or add validation which rejects configs which are not already in that format.

This is potentially a pretty big footgun for adopters of Relay. I myself hit it trying to setup a small project to repro an unrelated issue.

The invalid watchman file paths get constructed here: https://github.com/facebook/relay/blob/9b90142ae8135ac2bc5b42f9b6b093ba629beb0e/compiler/crates/relay-compiler/src/file_source/watchman_query_builder.rs#L56

Validating that the schema file exists is done here:

https://github.com/facebook/relay/blob/9b90142ae8135ac2bc5b42f9b6b093ba629beb0e/compiler/crates/relay-compiler/src/config.rs#L588-L599

Start by reading compiler/crates/relay-compiler/src/file_source/watchman_query_builder.rs around line 56 and compiler/crates/relay-compiler/src/config.rs around lines 588-599. Reproduce the configuration with a schema path beginning `./` while using Watchman, then trace the differing path handling. Done means the schema is discovered consistently and the compiler no longer panics.

Written by the indexing model from the issue text.