Import packages via sysl/pkg/mod package

[ChloePlanet]

Fixes #534 .

Changes proposed in this pull request:

• imported packages via sysl/pkg/mod package, which means:
  • by default:
  • //{github.com/anz-bank/pkg/foo.yaml@v0.0.1} downloads the v0.0.1 of pkg
  • //{github.com/anz-bank/pkg/foo.yaml@master} downloads pkg in master branch every time
  • //{github.com/anz-bank/pkg/foo.yaml} downloads and locks the package version the first time importing pkg.
  • arrai packages supports downloading single file via GitHub API by setting SYSL_MODULES=github
  • for more details, see: https://github.com/anz-bank/sysl/pull/993

Checklist:

• [x] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build 492bcf52d8730c4f6ab07935720b25885402bf17-PR-614

• 3 of 5 (60.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 48.546%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
syntax/import.go	3	5	60.0%
<!--	Total:	3	5	60.0%	-->

Files with Coverage Reduction	New Missed Lines	%
rel/value_set_generic.go	2	74.47%
rel/value_tuple.go	2	75.2%
<!--	Total:	4		-->

Totals
Change from base Build b168ef02be6ebd504934330e9ea4e264ef52c1ce:	-0.02%
Covered Lines:	4807
Relevant Lines:	9902

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💛 - Coveralls