Avoid processing the same EasyConfig multiple times

[Flamefire]

When passing the same EasyConfig file multiple times on the commandline it will be parsed once and put into a cache.
Every future call will get the same EasyConfig instance so any modification to it will be reflected in all future uses of supposedly "freshly parsed EasyConfigs".
This leads to confusing failures when trying to build the same file twice which is likely a mistake anyway, especially when one file is a symlink to another one passed too.
So just make sure each physical file is parsed only once.

I ran into this when building a software using usempi: True that errored during configure due to this:

== INFO Environment variable CXX set to mpicxx mpicxx (previously undefined)
== INFO Environment variable EBVARCXX set to mpicxx mpicxx (previously undefined)
== INFO Environment variable CXXFLAGS set to -O3 -march=native -fno-math-errno -g -O3 -march=native -O3 -march=native -fno-math-errno -fno-math-errno -g (previously undefined)

This is actually a bug introduced by 3836140:

• previously copy.deepcopy was used which would copy the easyconfig instances contained in the cached dict
• Now only a shallow copy of the dict is made, i.e. there is a separate instance of the dict but not of the keys and values

I don't think this was intentional, the commit message reads

use copy() method rather than copy.deepcopy on EasyConfig instances

But the operation is done on dict instances not EasyConfig instances. Shall we fix that too @boegel ?
That would we an alternative to this PR: If we do get independent EC instances then building the same EC twice should work. But I'd prefer this or both.

[boegel]

lgtm

[Flamefire]

Shall we fix the copy issue too? I.e. again use copy.deepcopy not (shallow) dict.copy? #4818