internal/filesystem: Account for case-insensitive filesystems

[radeksimko]

This was discovered during #771 but the problem existed prior to that PR and the PR isn't addressing it in any way.

terraform-ls/internal/filesystem/filesystem.go

Lines 210 to 238 in 6659c80

	func (fsys *fsystem) ReadDir(name string) ([]fs.DirEntry, error) {
	memList, err := afero.NewIOFS(fsys.memFs).ReadDir(name)
	if err != nil && !os.IsNotExist(err) {
	return nil, fmt.Errorf("memory FS: %w", err)
	}
	osList, err := afero.NewIOFS(fsys.osFs).ReadDir(name)
	if err != nil && !os.IsNotExist(err) {
	return nil, fmt.Errorf("OS FS: %w", err)
	}
	list := memList
	for _, osEntry := range osList {
	if fileIsInList(list, osEntry) {
	continue
	}
	list = append(list, osEntry)
	}
	return list, nil
	}
	func fileIsInList(list []fs.DirEntry, entry fs.DirEntry) bool {
	for _, di := range list {
	if di.Name() == entry.Name() {
	return true
	}
	}
	return false
	}

Background

Case-insensitive filesystems are relatively common (default) on Windows. They are however also commonly case-preserving. And given that it's (often? #75 ) the same OS running the client and the server, it is unlikely that we'd end up with two files which only differ in casing. That is likely why this never came up before.

Proposal

1. Understand if there are cases where OS of client can differ from OS of server, or the underlying filesystem (maybe in case of shared volumes?)
2. Check what other language servers do and try to understand why - i.e. what are the edge cases where the casing in file names can differ
3. Implement case-insensitive solution, if necessary - or document why we don't need to

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Related: spf13/afero#126