Add some more tests for config validation, and fix some gaps (#274)

I set these tests up in #270, but I realized there's a lot more we could
test unrelated to that PR. In this commit I add some more tests. They're
not really exhaustive yet, but they did catch a few bugs, which I fixed:
- we weren't validating the package-name if you do set it (only if we
guess it)
- If you omitted `generated`, you would try to write generated code to
the directory containing `genqlient.yaml`, which makes no sense; now we
default to `generated.go`. In the real world it's probably good to set
explicitly, but it's actually very convenient in tests that we don't
have to, and maybe in small projects too.

generate/config.go

@@ -145,6 +145,9 @@ func (c *Config) ValidateAndFillDefaults(baseDir string) error {
 	for i := range c.Operations {
 		c.Operations[i] = pathJoin(baseDir, c.Operations[i])
 	}
+	if c.Generated == "" {
+		c.Generated = "generated.go"
+	}
 	c.Generated = pathJoin(baseDir, c.Generated)
 	if c.ExportOperations != "" {
 		c.ExportOperations = pathJoin(baseDir, c.ExportOperations)
@@ -164,17 +167,23 @@ func (c *Config) ValidateAndFillDefaults(baseDir string) error {
 			"\nExample: \"github.com/Org/Repo/optional.Value\"")
 	}
 
-	if c.Package == "" {
+	if c.Package != "" {
+		if !token.IsIdentifier(c.Package) {
+			// No need for link here -- if you're already setting the package
+			// you know where to set the package.
+			return errorf(nil, "invalid package in genqlient.yaml: '%v' is not a valid identifier", c.Package)
+		}
+	} else {
 		abs, err := filepath.Abs(c.Generated)
 		if err != nil {
-			return errorf(nil, "unable to guess package-name: %v is not a valid identifier"+
+			return errorf(nil, "unable to guess package-name: %v"+
 				"\nSet package name in genqlient.yaml"+
 				"\nExample: https://github.com/Khan/genqlient/blob/main/example/genqlient.yaml#L6", err)
 		}
 
 		base := filepath.Base(filepath.Dir(abs))
 		if !token.IsIdentifier(base) {
-			return errorf(nil, "unable to guess package-name: %v is not a valid identifier"+
+			return errorf(nil, "unable to guess package-name: '%v' is not a valid identifier"+
 				"\nSet package name in genqlient.yaml"+
 				"\nExample: https://github.com/Khan/genqlient/blob/main/example/genqlient.yaml#L6", base)
 		}