solver3: Merge intersecting dependencies

If a package declares multiple dependencies that can be solved by
the same packages we should use the common set of packages to
solve them.

A common example is requiring the same Debian source version, or the
same upstream version as in our test case:

    git-ng Depends: git (>> 1:2.26.2), git (<< 1:2.26.2-.)

The solver expands this to the concrete objects:

    git-ng Depends: "real git" (= 1:2.26.2-1) | chaos-actor, "real git" (= 1:2.26.2-1) | "real git" (= 1:2.25.1-1)

When given an upgrade request, the solver would now choose

    chaos-actor               to satisfy git (>> 1:2.26.2)
    "real git" (= 1:2.25.1-1) to satisfy git (<< 1:2.26.2-.)

To satisfy the two constraints, which is not the intended outcome.

Address this problem by introducing a concept of merged clauses:
If two dependencies of a package have overlapping solutions, replace
the dependency by the intersection, and record the merged clause
instead, this leads to a single clause:

    Depends: git (>> 1:2.26.2) and git (<< 1:2.26.2-.)

which expands to just the real git binary.

The implementation is a bit finicky in that it removes the variables
from the original clause which may not be helpful for debugging, but
it records the clauses merged with, as seen in the test case, so the
reasoning is clear.

LP: #2111792

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