common/xbps-src: source all files under a directory named after the build-style.

[maxice8]

This allows a build-style to have various .sh under it, one important
use-case is sharing environment variables that can be set under multiple
build-styles.

As an example both cargo and meson build-style need to set various rust
flags, see #7322.

@Cogitri

[Vaelatern]

Interesting idea, I wonder if all possible entry points were covered, I'll grep for that.

[the-maldridge]

An interesting proposal, one that I'm not sure I fully understand the value. Please rebase to pick up the most recent fixes to the go style and then expound some upon the benefits of this strategy.

[Cogitri]

As explained before it's handy for stuff like #7322 where meson-rust needs the same env as cargo

[Gottox]

Nice idea!

I think this could be useful. We should introduce a build-style environment variable that points to $XBPS_COMMONDIR/build-style to ease linking and including other files. What do you think?

[Duncaen]

I don't think build styles need to be more complex.

The cargo configuration shouldn't depend on the build_style because at the end any buildstyle could need it, librsvg and firefox are examples that use no build_style and gnu-configure.

[maxice8]

superseeded by build_helper