wrong error when a part depends on a part with underscore

[Leo Arias]

I got into a weird situation where snapcraft tells me that a valid part name is invalid. After scratching my head for a while, I found that the invalid part name is a dependency. This misleading error is not fun.

To reproduce, write a snapcraft.yaml with the following parts:

parts:
  test-valid-part:
    plugin: nil
    after: [test_invalid_part]
  test_invalid_part:
    # See 'snapcraft plugins'
    plugin: nil

Run snapcraft:

$ snapcraft
Issues while validating snapcraft.yaml: The 'parts' property does not match the required schema: 'test-valid-part' is not a valid part name. Part names consist of lower-case alphanumeric characters, hyphens, plus signs, and forward slashes. As a special case, 'plugins' is also not a valid part name.

Notice that the error refers to the valid part.