Fix canonicalize_resource to only match resources with same namevar

[domcleal]

9341e04 introduced a regression when using matchers to check that a
resource reference doesn't exist, as the matcher will find a resource of
the same type but with a different namevar/title.

The following example would fail if any Bar resource exists, as the
title in the Bar[unknown] resource reference would be ignored:

is_expected.not_to contain_foo("foo").that_requires("Bar[unknown]")

The method passed a block to #first rather than #find in order to filter
by the namevar, but the block is unused.

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Fix nil error for irretrievable Class[main] resource

When iterating over @catalogue.resource_keys, a builtin Class[main]
resource is returned, but isn't accessible through #resource, causing
a nil entry in the list of matched resources. The array of resources is
now compacted to remove inaccessible builtin resources, which fixes an
error when using Class resources in relationship matchers.

[coveralls]

Changes Unknown when pulling fbac738 on domcleal:fix-negative-relationship-matchers-find-by-title into ** on rodjek:master**.