look up constraints using their canonical name

[dhellmann]

Authors of requirements and constraints files are not consistent in how they capitalize or punctuate the package names. Convert the names to their canonical forms in the dictionary and when we try to look up a package so the names will match more often. Change the API of Constraints.get_constraint() to only take a name so we can do the lookup more consistently.

[dhellmann]

@Mergifyio rebase

[mergify[bot]]

rebase

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