store all patches (incl the ones for extensions) that need to be copied to a repo/reprod dir in an all_patches set

[bedroge]

This also solves the duplication issue that #4959 tried to solve, which may cause issues for anyone trying to install an easyconfig that has an extension with a patch, and when that patch has read-only file permissions (this may happen frequently if EasyBuild itself was installed with --read-only-installdir).

Note that patches of components are added to self.patches by the Bundle easyblock: https://github.com/easybuilders/easybuild-easyblocks/blob/develop/easybuild/easyblocks/generic/bundle.py#L222. Hence, they will also be included in self.all_patches.
Do note that, regardless of the change in this PR, the approach used by the Bundle may actually cause issues, because sources/patches may then be expected by the main easyblock itself, and they may also end up in the easyconfig stored in the reprod folder (see #4862). However, I felt that solving that was out of scope for this PR, and could be handled in a follow-up PR.

edit: discussed this a bit more with @ocaisa on Slack, and converted all_patches into a set containing FrozenDicts. This prevents that duplicate entries may show up in the list, i.e. it would prevent that due to future code changes the same issue as #4959 would pop up again. When I tried to use FrozenDicts, I got this error:

    self.__hash = reduce(operator.xor, map(hash, self.iteritems()), 0)
                                                 ^^^^^^^^^^^^^^
AttributeError: 'FrozenDict' object has no attribute 'iteritems'

Solved this by changing self.iteritems() to self.items().

edit2: we actually only need to know the paths to all patches, so let's only store those instead of the entire patch_info dictionary.

[ocaisa]

This looks very clean now, if the existing tests (https://github.com/easybuilders/easybuild-framework/blob/develop/test/framework/toy_build.py#L1390) pass I think this is good to go

[ocaisa]

LGTM