Support strict build dependencies (bst 1)

Tristan Van Berkomrequested to merge
tristan/bst-1/strict-rebuild into bst-1

This fixes a long time imperfection in non-strict build semantics, allowing one to declare specific dependencies which will cause reverse dependency rebuilds unconditionally, useful for cases when an element might static link to a dependency in a non-strict build plan.

This fixes #254 (closed) and is a backport of !1542 (merged)

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