Move VERSION file to src folder

[tisonkun]

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• I had searched in the issues and found no similar issues.

Motivation

Move VERSION file to src folder, so that we have a tidy root layout.

Solution

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!

[tisonkun]

You should correspondingly update the build logic to fulfill VERSION information, including those in x.py and CMakeList.txt.

[PragmaTwice]

Actually I have some doubt about this proposal. In my view, the VERSION file is not a source file, which seems not proper to be placed into the src directory.

We can see the CI failed when someone try to move this file to src in #1149, whose reason is quite funny: there is a header named version in the C++ standard, and the move cause compiler to find wrong header and fail to compile.