exec: fix case-insensitive file name collision

[grzn]

Similar to #2829.

1. added a case under codegen/testserver/casesensitivity/ to demonstrate the problem
2. added handing of filenames with different cases, similar to what was done in resolver: fix case-insensitive file name collision #2829

[StevenACoffman]

Suggested change

	func addBuild(fnCase string, fnKey string, p *ast.Position, data *Data, builds *map[string]*build) {
	func addBuild(fnCase, fnKey string, p *ast.Position, data *Data, builds *map[string]*build) {

[StevenACoffman]

The abbreviation you chose fn for fnCase and fnKey is commonly used to mean function so it may be a little confusing here?

[StevenACoffman]

Also, even in gigantic projects, the number of files is probably not significant, as you do in a number of the other functions, so I'm not sure that we need to bother passing the same data fnCase and fnKey here, when we can just re-compute fnKey from strings.ToLower(filename)

[StevenACoffman]

Hi! I appreciate your PR, but the fmt-and-lint check failed with:

Error: generate: open /home/runner/work/gqlgen/gqlgen/codegen/testserver/casesensitivity/resource.generated.go: no such file or directory
Error: Process completed with exit code 1.

And when I looked at the lint failures, I questioned whether we need to change that signature at all, since the strings.ToLower() conversion of a filename is not going to make a measurable performance impact at the scale of the number of files in the vast majority of projects.

BTW, https://github.com/SimonWaldherr/golang-benchmarks#caseinsensitivecompare shows that strings.ToUpper() is faster and has fewer allocations.