Merge branch 'edbaunton/remote-source' into 'master'

buildstream/_versions.py

buildstream/plugins/sources/remote.py

doc/source/core_plugins.rst

tests/sources/remote.py

tests/sources/remote/missing-file/target.bst

tests/sources/remote/no-ref/file

tests/sources/remote/no-ref/target.bst

tests/sources/remote/path-in-filename/dir/file

tests/sources/remote/path-in-filename/target.bst

tests/sources/remote/single-file-custom-name/dir/file

tests/sources/remote/single-file-custom-name/target.bst

tests/sources/remote/single-file/dir/file

tests/sources/remote/single-file/target.bst

tests/sources/remote/unique-keys/dir/file

tests/sources/remote/unique-keys/target-custom.bst

tests/sources/remote/unique-keys/target.bst