FindProjectDir return the first path that contains go.mod.

/a/b/c -> /a ok /a/b/c -> /am !ok /a/b/c/ -> /a/b/c ok /a/b/cd -> /a/b/c !ok.

NOTE: a replacement of implements.

Module is nil, and ends with .test or _test.

NextName `addIfNotExists` returns true if `s` available.

RelPath returns "" if child is not in base.

if pathName is "", cwd is returned.