tests/cachekey: Test cache keys are independent of target elements

See merge request !1123

Improve our filter documentation

Closes #278

See merge request !1112

This patch also uncovered the fact that our test_filter_deps_ok() test
has been inaccurate. Thus the element built in this test
(deps-permitted.bst) has been modified so that it build depends on the
input.bst element, as it should.

tests/filter.py: Ensure deps_ok test passes

Fail when we explictly try to include/exclude non-existent domains in a filter element

See merge request !1117

The unix specification for unix sockets only allocates ~100 characters
for the length of the path. This may be longer than the path for a CI
runner, or the path to a user's buildstream directory.