Support constraint files

[mkniewallner]

pip supports setting constraint files in requirements files (e.g, -c constraints.txt) to set specific versions constraints on some dependencies. We could parse -c <constraints_file> in requirements files, and if found:

• extract constraints from the files
• set those constraints to tool.uv.constraint-dependencies in get_constraint_dependencies before locking
• remove constraint files in get_migrated_files_to_delete

Note that like -r <requirements-file>, it's possible to set the value without any whitespace, so we'd also need to parse -cconstraints.txt.