Script startup latency improvement

[philwalk]

When launching a scala3 script, there are two calls to dist/bin/common, the first from within dist/bin/scala and the 2nd when dist/bin/scala calls dist/bin/scalac.

It turns out that the avoidable 2nd call is quite expensive in terms of what it adds to script startup latency. Here are the reductions in startup time when running in cygwin and in WSL Linux:

cygwin:    837 mSec
WSL Linux:  66 mSec

The numbers are the difference in startup latency for calling a hello-world.sc script with and without the changes in this PR, averaged over 100 runs.

The categories of change in the PR are:

• exporting values initialized by dist/bin/common and required by scalac
• modification of dist/bin/scalac to conditionally source dist/bin/common
• changes to avoid spurious unset variable errors when testing with: set -o nounset
• fail early if a script file doesn't exist

The third change is exemplified by line 114 in dist/bin/common:

-if [ -n "$CYGPATHCMD" ]; then
+if [ -n "${CYGPATHCMD-}" ]; then

[philwalk]

Unintentionally included the changes from #11633, I'll have to re-create this PR later.

[griggt]

Just an FYI, I've noticed that the act of re-opening a PR launches a fresh CI run. There are 3 jobs now running in CI for this PR:

https://github.com/lampepfl/dotty/actions/runs/633879155
https://github.com/lampepfl/dotty/actions/runs/633879159
https://github.com/lampepfl/dotty/actions/runs/633891730

[philwalk]

@griggt - good to know, will avoid this in the future.

[philwalk]

This PR needs to be re-created from scratch, to avoid possible problems with #11633

[philwalk]

This PR is superceded by #11698