`LocalPath`: resolve to `AbsolutePath`

[ForNeVeR]

We need a function that will take a LocalPath and convert it into an AbsolutePath, by these rules:

• if the path is already absolute, return as-is (re-wrapping into an AbsolutePath)
  • consider the optimization mentioned in #17
• if the path is relative, then resolve it relatively to the base directory
  • the base directory may be passed or may be the CWD by default (I would like this variant to be explicit, though, perhaps another overload or even a new name)