precompile: if a single package is requested, show output live

[IanButterworth]

Fixes #3487

@vtjnash something like this, perhaps?

[IanButterworth]

An alternative would be for code loading precompilation to parallel precomp all of the depdendencies but leave off the requested package, then exit and leave the old serial precompilation to finish the main package like it used to.
Though it would look a bit like a bug if it did that, I think.

[vtjnash]

Seems useful, but also a bit odd to see the output repeated twice. I like the concept of having output appear as each task finishes for each process. For comparison, in gmake, this would correspond to the --output-sync={none,line,target,recurse} options, of which my preferred is usually target and that is also the default. Currently we instead implement the equivalent of recurse only.

[IanButterworth]

Updated to

Note that now if you call precompile in a package project, it will follow this behavior too.

[IanButterworth]

I like the concept of having output appear as each task finishes for each process.

It's an interesting idea, but would be a bigger change with some tricky UX because of the way packages disappear when done. This PR is a bit more manageable in UX because it only happens to the last singular package being precompiled.

[IanButterworth]

Marking this for 1.10 backport because the reported issue was introduced by code load parallel precompile which was introduced in 1.10

[IanButterworth]

Let's get this on master to get it tried out