Properly handle libraries with spaces in names, when installed as custom libraries

[ianfixes]

If a library with a space in the name is requested, we expect it to be stored on disk as having an underscore substituted for the space. Detection of already-installed libraries breaks somewhat in this case. Consider the following:

unittest:
  platforms:
    - mega2560
  libraries:
    - "Adafruit BusIO"
    # - "Adafruit_BusIO"    <= This works if you have the library pre-installed