warn instead of error for pinned versions

[WebReflection]

The goal of this MR is to fix #161 however I think it's optimistic to believe Pyodide and its micropip related to that pyodide could do anything about packages not compatible/available for a specific Pyodide version.

The thing is, if I force version 0.28.0 which is not compatible with pygame-ce there's apparently no way to force loading a previous version of pygame-ce even if it used to be compatible.

This makes our discussion around pinned versions mostly irrelevant, although the logic to warn instead of always throwing is in place, I really would like to have a concrete example of when that warning should be read but the older package should still work (this is in particular for @mchilvers and @fpliger)

/cc @ntoll as it looks that we cannot control much of this dance, I might ping Hood around this topic too (he is not watching this space, it'll happen on discord).

Current State

• if a package is pinned and its version is available somewhere, a warning is shown that such version might not work
• if a package is pinned and its name is unknown, we throw an error as previously discussed and agreed on
• in every other case we let Pyodide and micropip sort everything out ... we don't get to decide how Pyodide or micropip work in there, also there are limitations with pinned version and the cache used internally, so basically we're always better off without pinned versions as each Pyodide version already guarantees that pinned version so this whole dance might be overkill in general (still fast right now, not a perf issue, rather a maintenance one).