fix: clarify the use of SnapcraftConfig

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

• Use a more recommended type definition for SnapcraftConfig
• Clarify the difference between SnapcraftConfig and Options.appConfig

[codecov]

Codecov Report

Merging #76 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #76   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          252       252           
=========================================
  Hits           252       252           

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