Honor cpanfile requirements when recursing sub dependencies

[miyagawa]

This will save the parsed cpanfile requirements during the install session, and when extracting sub dependencies, cpanm will merge the prereqs of the root cpanfile, so that requirements in cpanfile will still effect there.

This will address #363 and lots of issues with Carton such as perl-carton/carton#68

[miyagawa]

Most tests failures on Travis seem like due to errors on metacpan side randomly. Tests are all passing locally.

[skaji]

I'm not sure this shoud be fixed or not, but comment here.

If cpanfile is:

requires 'Module::Refresh', '== 0.17';
requires 'Path::Class::File', '== 0.25';

then,

$ cpanm -n -Llocal --installdeps .
--> Working on .
Configuring /Users/skaji/work/2015-04-12 ... OK
==> Found dependencies: Module::Refresh, Path::Class::File
--> Working on Module::Refresh
Fetching http://www.cpan.org/authors/id/A/AL/ALEXMV/Module-Refresh-0.17.tar.gz ... OK
Configuring Module-Refresh-0.17 ... OK
==> Found dependencies: Path::Class
--> Working on Path::Class
Fetching http://www.cpan.org/authors/id/K/KW/KWILLIAMS/Path-Class-0.35.tar.gz ... OK
==> Found dependencies: Module::Build
--> Working on Module::Build
Fetching http://www.cpan.org/authors/id/L/LE/LEONT/Module-Build-0.4211.tar.gz ... OK
==> Found dependencies: CPAN::Meta
--> Working on CPAN::Meta
Fetching http://www.cpan.org/authors/id/D/DA/DAGOLDEN/CPAN-Meta-2.150001.tar.gz ... OK
Configuring CPAN-Meta-2.150001 ... OK
Building CPAN-Meta-2.150001 ... OK
Successfully installed CPAN-Meta-2.150001 (upgraded from 2.140640)
Configuring Module-Build-0.4211 ... OK
Building Module-Build-0.4211 ... OK
Successfully installed Module-Build-0.4211 (upgraded from 0.4205)
Configuring Path-Class-0.35 ... OK
Building Path-Class-0.35 ... OK
Successfully installed Path-Class-0.35
Building Module-Refresh-0.17 ... OK
Successfully installed Module-Refresh-0.17
--> Working on Path::Class::File
Fetching http://www.cpan.org/authors/id/K/KW/KWILLIAMS/Path-Class-0.25.tar.gz ... OK
Configuring Path-Class-0.25 ... OK
Building Path-Class-0.25 ... OK
Successfully installed Path-Class-0.25 (downgraded from 0.35)
<== Installed dependencies for .. Finishing.
5 distributions installed

=> install Path-Class distribution twice

[miyagawa]

that is definitely a corner case, but also a correct behavior, because Module::Refresh wants Path::Class whatever version, so cpanm will get the latest from CPAN, then found that you want Path::Class::File == 0.25, so it tries to downgrade later.

It's absolutely a correct behavior. It would be nicer if the actual installation happens later, after building the whole dep tree, but that's a very big change required for the cpanm installation process.