Allow const as argdep

[csweichel]

Description

Prior to this change the following component would not build without passing foobar as build arg:

const:
  - foobar=baz
packages:
  - name: demo
    type: generic
    argdeps:
      - foobar

[codecov]

Codecov Report

Merging #164 (874802a) into main (e591a13) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##            main    #164      +/-   ##
========================================
- Coverage   8.36%   8.36%   -0.01%     
========================================
  Files         20      20              
  Lines       3406    3409       +3     
========================================
  Hits         285     285              
- Misses      3069    3072       +3     
  Partials      52      52