Add support for parsing dependencies from *.nimble files

[andrew]

Details: https://github.com/nim-lang/nimble#creating-packages

Example: https://github.com/nim-lang/nimble/blob/master/nimble.nimble

import ospaths
template thisModuleFile: string = instantiationInfo(fullPaths = true).filename

when fileExists(thisModuleFile.parentDir / "src/nimblepkg/common.nim"):
  # In the git repository the Nimble sources are in a ``src`` directory.
  import src/nimblepkg/common
else:
  # When the package is installed, the ``src`` directory disappears.
  import nimblepkg/common

# Package

version       = nimbleVersion
author        = "Dominik Picheta"
description   = "Nim package manager."
license       = "BSD"

bin = @["nimble"]
srcDir = "src"

# Dependencies

requires "nim >= 0.13.0", "compiler#head"

when defined(nimdistros):
  import distros
  if detectOs(Ubuntu):
    foreignDep "libssl-dev"
  else:
    foreignDep "openssl"

task test, "Run the Nimble tester!":
  withDir "tests":
    exec "nim c -r tester"