pluginhandler: add support for disabling system library migration

[kyrofa]

Currently snapcraft, during the prime step for a part, checks to see if that part needs any libraries from the system. This is not always desired, as in some cases the libraries will be provided by another snap via the content interface.

This PR fixes LP: #1639831 by introducing a new property to the base part YAML: build-attributes. It's simply a list of build attributes, only one of which exists right now: no-system-libraries. For example:

parts:
  # ...
  example:
    plugin: cmake
    build-attributes: [no-system-libraries]
  # ...

Using the no-system-libraries attribute has the effect of not pulling any libraries from the system during the prime step for that part.

[sergiusens]

More than build-options I'd call these build-attributes

[kyrofa]

More than build-options I'd call these build-attributes

Works for me-- done.

[codecov-io]

Current coverage is 96.40% (diff: 100%)

Merging #989 into master will increase coverage by <.01%

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  Files           190        192     +2   
  Lines         16926      16962    +36   
  Methods           0          0          
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  Branches       1292       1293     +1   
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+ Hits          16316      16352    +36   
  Misses          418        418          
  Partials        192        192          

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[sergiusens]

Looks good, the in-code and md documentation needs to be updated to add this though.

[kyrofa]

Looks good, the in-code and md documentation needs to be updated to add this though.

Ah of course! Done.