Interpret paths in diesel.toml as relative to the project root

[sgrif]

I went ahead and also changed the search for project root to look for
diesel.toml first, which should improve our behavior in workspaces.

Fixes #2057.

[JohnTitor]

Diesel CLI test has been failed in this assertion. It seems that the path was changed.

diesel/diesel_cli/tests/migration_run.rs

Lines 502 to 508 in fcf690d

	assert!(
	result
	.stderr()
	.contains("Command would result in changes to src/my_schema.rs"),
	"Unexpected stderr {}",
	result.stderr()
	);

[AlterionX]

Just a couple of nits.

[AlterionX]

I think that this clarification makes it a little more confusing. It might be more effective to state in the previous paragraph what the project root is, then just reuse the term, especially since there can be multiple Cargo.toml files (which this PR is trying to fix anyways).

[AlterionX]

It may be useful to indicate what "this" directory is. I don't know much about efficiency if you include a format! here, though. That would entail passing in either the url into the error, or something similar, which might cause enum size differences? But I'm not sure.

[sezna]

I was looking on the pipeline to see what failed and Azure says the build wasn't found -- does anybody know why it failed the pipeline?