Add optional "minimum required versions" to smackspec requires, use it to install packages faster

[robsimmons]

In addition to the existing smackspec syntax

require: cmlib v3

which specifies that the user will only refer to "v3", it would be helpful to have a "minimum" syntax, like this:

require: cmlib v3 (v3.19.12)

which says that v3 will satisfy their constraints, but that it must be at least v3.19.12 (this only makes sense when the constraint - v3 in this case - satisfies the minimal version - v3.19.12 in this case). This is the same as the common semantic version constraint pattern "<4.0.0 >=3.19.12" seen in other tools.

Once this syntax was in place, the "resolve dependencies" code could be made slightly more complicated in order to notice the case where, while 3.19.12 is required and 3.30.0 is available, 3.24.4 is already installed in the system, so there's no need to get the most recent version.

[Ahnfelt]

Perhaps since we're using semantic versioning,

requires: cmlib 3.19.12

should always mean

requires: cmlib >=3.19.12 <4.0.0

If the semantic versioning is followed perfectly, then all later versions with the same major version number should be compatible.

This may require the addition of equality constraints for those cases where semantic versioning wasn't followed, eg.

requires: cmlib ==3.19.12

[robsimmons]

My thought about the current design is that the thing you write after requires: cmlib (either v1 or v1.3 or v1.3.5) is connected to the file path that you use to access cmlib, so if you write v1.3, you don't care if smackage, behind the scenes, deletes v1.3.5 and replaces it with v1.3.6, but that if v1.5.7 gets installed you expect some variant of v1.3 to stick around. That's been my rationale all along for having a single specific constraint like v1.

[Ahnfelt]

That makes sense; I guess I'm still thinking in Hackage terms.

[robsimmons]

One of the many things done in commit 50e816a was to add this syntax, which can support the short-cut install.