Manifests reachable by NTT_CACHE overwrite "." root folders

[Matthias Simon]

Describe the bug
ntt can't load test-suite from directory ., when a manifest file is already reachable by NTT_CACHE environment variable.

To Reproduce

# Initial setup:
# The temporary directory contains no package.yml. Therefore ntt is expected to use the directories base name as test suite name:
$ cd $(mktemp -d)
$ ntt show . -- name 
tmp_ngu9w0zasp

# Bug:
# When there's a package.yml reachable by NTT_CACHE, ntt will ignore the base-name directory
$ mkdir some_dir
$ echo "name: bad" > some_dir/package.yml
$ NTT_CACHE=some_dir  ntt show . -- name 
bad

# Weird:
# NTT_CACHE is ignored and the expected test suite name is shown again, if there's a package.yml in current directory.
$ touch package.yml
NTT_CACHE=some_dir  ntt show . -- name
tmp_ngu9w0zasp

Expected behavior
When we ask ntt to show configuration of "." we expect to use this and only this path.

Possible fixes
We should not use NTT_CACHE mechanism, when a path starts with ".". The filepath.Join function however removes any dots:

$ cat main.go
package main

import (
	"fmt"
	"path/filepath"
)

func main() {
	fmt.Println(filepath.Join("./foo/", "package.yml"))
}

$ go run
foo/package.yml

One possible fix would be not to use join and let ntt clean the path later.