[WIP] make install a command generator

[chasinglogic]

This is an idea we've had for a long time. It allows for very interesting use cases like adding --reflink=auto for faster installs on FS that support it, using symlinks for installs for developer builds, and perhaps the most interesting:

env.Install(
    target="$DEPLOY_DIR",
    source=myapp,
)

env['COPY'] = 'rsync'
env['COPYFLAGS'] = ['-avz']

python ./buildscripts/scons.py DEPLOY_DIR="dev-server:/app/deploy" deploy

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated master/src/CHANGES.txt directory (and read the README.txt in that directory)
• I have updated the appropriate documentation

[bdbaddog]

I'd like to see the variables all be INSTALL_ or INSTALL* instead of COPY* affecting Install().

Any idea if this impacts performance? (I'd expect it to be faster and not subject to GIL..)

[bdbaddog]

But hold off on changes to name. Let's catch up on discord tomorrow.

[bdbaddog]

Also.. doc changes will be needed.

[bdbaddog]

Closing this as work moved to PR #3573