Allow some package information to be fetched externally

The original "makepkg" will reset all package information, and that diasllows
to support some hooks (no we do not have any hook when using PKGBUILD.)
Accept checksum (512) from environment
pkgbase will define the behavior

As "pkgbase" is a string, while "pkgname" may be an array,
we can only get "pkgbase" from environment.
makepkg accepts PACKAGE_FEATURE

This variable is used to control some feature of output filename,
and of the package information
Add PACKAGE_FEATURE to some output message
Feature string will start with @

Using a dash (-) will not make thing clear. We use @
to provide a feature string for any package. Example,

  pacman@some-special-feature
  uim-vi@with-more-clean

TODO: I do not really know if :@ would break something,
but so far I have no problem with that. New package works
PACKAGE_FEATURE will only alter $conflicts

Previously, the feature will alter $provides, $replaces.
However, $replaces is completely wrong. $provides may not
be always correct. $conflicts is enough. However, see
next commits and features
Use the original $PACKAGE_CONFLICT_TYPE in _FEATURE_STRING

Because the modifier now has a mean (@ is not = / %), we must keep
the modifier in the _FEATURE_STRING.
Not as said in #cff343f, we need to keep $provides variable

That allows group can use feature package instead of the origin package
Do not use PACKAGE_CONFLICT_TYPE to get conflict type
Load feature PKGBUILD if any

For the original package, PGKBUILD is used by default.
If there is a feature, e.g, @foobar, we will check if
there is a file PKGBUILD@foobar, and load that file.
This way we do not need to edit the original file, bc.
everything can be overwritten in the feature file.

Is that good?
Use TheSLinux source mirror by default

All source files will be downloaded from #theslinux mirror;
The list of URIs is returned by (_pkgbuild_s_sources).
To use the original PKGBUILD sources, set the variable
WITHOUT_THESLINUX_SOURCES=1
Skip checksums by default