CLI feature: aliases for JDK programs

[marktsuchida]

Instead of cjdk exec java, allow just cjdk java.

Do this for all programs in the selected JDK's bin directory.

• This saves keystrokes
• We can guarantee that the executable invoked is exactly the one in the JDK, preventing any leak to something further down PATH.

[marktsuchida]

Need some thought about how to do this when the JDK is not yet cached: do we download the JDK for any unknown subcommand? Or just hard-code the known programs usually found in $JAVA_HOME/bin? Probably the latter.

[marktsuchida]

On the other hand, the only real benefit here is the check for the program being part of the JDK. This could be done simply with a flag to exec: --jdk-bin.

Instead, we can add a java command with special functionality, now that we have cache_file: java could invoke the JVM after replacing, in its arguments, https JAR URLs with cached local paths (for -jar and -cp).

Programs other than java are likely used a lot less frequently, especially in manual command-line invocation, so exec --jdk-bin is probably good enough.

[ctrueden]

The ones I use a lot are java, javac, javadoc, javap, jshell. There is also javah but I almost never use it, personally. I think an 80/20 solution here is to just hardcode these. To run others, there is still exec.

We can guarantee that the executable invoked is exactly the one in the JDK, preventing any leak to something further down PATH.

Isn't this (or couldn't it be?) a benefit of exec generally, even without these shortcuts?