I've made a number of fixes to the mvs package for Go 1.17, but didn't file an issue for them yet because they were discovered incidentally to #36460.

I'm filing this issue mostly so that I have something to point to if someone notices these bugs in previous versions, so that they can tell when the bugs were fixed.

There were two bugs affecting cmd/go:

• CL 294294: mvs.Downgrade could introduce spurious dependencies due to the downgraded modules being upgraded to “hidden” versions (such as pseudo-versions) by other downgraded dependencies.
• CL 290770: The combination of mvs.Upgrade and mvs.Downgrade could introduce spurious new or upgraded dependencies, introduced as transitive dependencies of upgraded modules that were subsequently downgraded.

In addition, there were fixes for three mvs bugs that did not affect cmd/go due to how cmd/go was invoking mvs:

• CL 266339 (Go 1.16): mvs functions sometimes did not omit modules whose selected version is none.
• CL 287632: mvs.Req did not suppress duplicate paths in the base argument.
• CL 287633: mvs.Downgrade did not retain certain transitive dependencies.