Add validation for Kitfiles

[amisevsk]

Description

Add some basic validation to Kitfiles, printing an error if

• Multiple layers share the same path (makes unpacking/ignoring unclear)
• Any layer is an absolute path rather than relative
• Model part's type field is invalid: (max 64 characters, alphanumeric with ., -, and _; must start with alphanumeric)

Testing

You can see all of the failure cases by trying to pack the following Kitfile

Kitfile

manifestVersion: 1.0.0
package:
  name: test-kf
model:
  name: my-model
  path: path1
  parts:
    - name: part1
      path: path1
    - name: part2
      path: /abs/path
    - name: part3
      path: valid-path
      type: test type with space
    - name: part4
      path: also-valid
      type: really-long-but-otherwise-valid-type---this-line-is-65-chars-long
datasets:
  - name: dataset1
    path: path1
  - name: dataset2
    path: path3
  - name: dataset3
    path: /abs/path/2
code:
  - path: path1
  - path: path3
  - path: /abs/path/3

Linked issues

Closes #81

[gorkem]

Why 64?

[amisevsk]

Arbitrary choice -- big enough that nobody should hit the limit but also not too big since we can increase this without breaking any Kitfiles.